I wrote an apex class which works fine. However, the test class keeps failing with the error: System.EmailException: SendEmail failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Email body is required.: [] Not sure what I am doing wrong. Can you please help? I have attached my code and test class below

global class XYZLetter {   
    global static void sendLetter(List<String> recordId) {
        
//Get Files from ContentDocumentLink Object
    List<ContentDocumentLink> cdls = [select Id,ContentDocumentId,ContentDocument.Title,LinkedEntity.Name From ContentDocumentLink where LinkedEntityId=:recordId AND ContentDocument.Title LIKE 'xyz%'];
    
    Set<Id> contentDocumentIds = new Set<Id>();
    for (ContentDocumentLink cdl : cdls) {
        contentDocumentIds.add(cdl.ContentDocumentId);
    }   
        
    List<Id> attachmentIds = new List<Id>();    
//Get all related contentVersion file     
    List<ContentVersion> documents = ([SELECT Id FROM ContentVersion WHERE isLatest = true AND ContentDocumentId IN :contentDocumentIds]);
    for (ContentVersion doc : documents) {
        attachmentIds.add(doc.Id);
    }
    
//Get details on the Application    
   Card__c pcard = [SELECT Id,Letter_Failed__c, RecordTypeId FROM Card__c WHERE Id=:recordId];
   
    String cardholderContactId=pcard.Card_Applicant__c;

//-********************
 
    if(attachmentIds.size()==0){
        EmailTemplate NoXYZ = [SELECT id FROM EmailTemplate WHERE developerName = 'Letter_Not_Found'];
        
        OrgWideEmailAddress[] owea = [select Id from OrgWideEmailAddress where Address = 'oxyxSF@COMPANY.COM'];
        String[] ccAddresses = new String[] {'cardinquiries@company.com'};
        
        //Apex Single email message
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        if ( owea.size() > 0 ) {
            mail.setOrgWideEmailAddressId(owea.get(0).Id);
        }    
      
        mail.setTemplateID(NoXYZ.id);//templateID
        mail.setTargetObjectId(cardholderContactId);// contact Id of cardholder
        mail.setCcAddresses(ccAddresses);
        //No attachment found here, so no need to try and attach mail.setEntityAttachments(attachmentIds);
        mail.setWhatId(pcard.Id);  
        Messaging.SingleEmailMessage[] messages = new List<Messaging.SingleEmailMessage> {mail};
        Messaging.SendEmailResult[] results = Messaging.sendEmail(messages);
            if (results[0].success) {
                System.debug('The email for XYZ Letter not found was sent successfully.');
            } else {
                System.debug('The email for XYZ Letter not found failed to send: '+ results[0].errors[0].message);
                 pcard.XYZ_Letter_Failed__c = true;
             }
     
              Update pcard;
   }//end if attachmentIds.size()==0 (Send 'XYZ Letter Not Found' Template end)
//-********************

//-********************
   //Otherwise, end message as expected
    else {
        Id pcardRTypeId = [SELECT Id FROM RecordType WHERE sObjectType='Card__c' AND developerName ='PCard_Approval'].Id;
        Id finRTypeId = [SELECT Id FROM RecordType WHERE sObjectType='Card__c' AND developerName ='Information_Updates'].Id;

        EmailTemplate pcardET = [SELECT id FROM EmailTemplate WHERE developerName = 'X3_Approved'];
        EmailTemplate finET = [SELECT id FROM EmailTemplate WHERE developerName = 'X20_Information_Update_Approved'];
        EmailTemplate SingleFinET = [SELECT id FROM EmailTemplate WHERE developerName = 'X201_Single_Information_Update_Approved'];
        EmailTemplate MonthlyFinET = [SELECT id FROM EmailTemplate WHERE developerName = 'X202_Monthly_Information_Update_Approved'];
        
        String et;
        if (pcard.RecordTypeId == pcardRTypeId){
            et = pcardET.Id;
        }
        else if (pcard.RecordTypeId == finRTypeId){
            //et = finET.Id;
            if (pcard.Monthly_Purchase_Card_Update__c && pcard.Single_Purchase_Card_Update__c){
                et = finET.Id;
            }
            else if(pcard.Monthly_Purchase_Card_Update__c){
                et = MonthlyfinET.Id;
            }
            else if(pcard.Single_Purchase_Card_Update__c){
                et = SingleFinET.Id;
            }
        }

       
        

        OrgWideEmailAddress[] owea = [select Id from OrgWideEmailAddress where Address = 'XYZsalesforce@COMPANY.COM'];
  
        String[] ccAddresses = new String[] {'p-cardinquiries@COMPANY.COM'};
      
        if (pcard.PCard_Supervisor__c != null){
           ccAddresses.add(pcard.PCard_Supervisor__c);
        }
        if (pcard.Approving_Official_Lookup__c != null){
           ccAddresses.add(pcard.Approving_Official_Lookup__c);
        }
        
        List<String> idList = new List<String>();
        Group g = [SELECT (select userOrGroupId from groupMembers) FROM group WHERE NAME =:pcard.CPublic_Group__c];
        for (GroupMember gm : g.groupMembers) {
            idList.add(gm.userOrGroupId);
        }
        
        User[] usr = [SELECT email FROM user WHERE id IN :idList];
        for(User u : usr) {
            ccAddresses.add(u.Id);
        }
  
    //Apex Single email message
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        if ( owea.size() > 0 ) {
            mail.setOrgWideEmailAddressId(owea.get(0).Id);
        }    

    mail.setTemplateID(et);//templateID
    mail.setTargetObjectId(cardholderContactId);// contact Id of cardholder
    mail.setCcAddresses(ccAddresses);
    mail.setEntityAttachments(attachmentIds);
    mail.setWhatId(pcard.Id);  // card recordId
        
    Messaging.SingleEmailMessage[] messages = new List<Messaging.SingleEmailMessage> {mail};
    Messaging.SendEmailResult[] results = Messaging.sendEmail(messages);
        if (results[0].success) {
           System.debug('The email was sent successfully.');
        } else {
           System.debug('The email failed to send: '+ results[0].errors[0].message);
        }

        
    }

Here is the test class:
@isTest
public class DPALetterTest {
    static testMethod void sendEmailwithAttachment() {
        Test.startTest();
        
        EmailTemplate et = [SELECT id FROM EmailTemplate WHERE developerName = 'X3_PCard_Approved'];
        Id pcardRTypeId = [SELECT Id FROM RecordType WHERE sObjectType='PCard__c' AND developerName ='PCard_Approval'].Id;
        Id finRTypeId = [SELECT Id FROM RecordType WHERE sObjectType='Purchase_Card__c' AND developerName ='Information_Updates'].Id;
       // String pcardRTypeId = recTypeId.Id;
        
        Account acc = new Account();
        acc.Name='ABCTest';
        insert acc;

        String profileName1 = 'Card Application User';
        
        User admin = [SELECT Id, Username, UserRoleId FROM User WHERE Profile.Name = 'System Administrator' AND UserRole.Name='XYZSystem Admin' LIMIT 1];
       
        System.runAs(admin){

            User u1 = new User(
                ProfileId = ([SELECT Id FROM Profile WHERE Name=:profileName1].Id),
                LastName = 'user1',
                Email = 'test.user1@test.com',
                Username = 'test.ccpuser1@test.com',
                CompanyName = 'XYZ',            
                Alias = 'alias',          
                TimeZoneSidKey = 'America/Los_Angeles',
                EmailEncodingKey = 'UTF-8',
                LanguageLocaleKey = 'en_US',
                LocaleSidKey = 'en_US'             
            );
            insert u1;

            User u2 = new User(
                ProfileId = ([SELECT Id FROM Profile WHERE Name=:profileName1].Id),
                LastName = 'user2',
                Email = 'test.user2@test.com',
                Username = 'test.ccpuser2@test.com',
                CompanyName = 'CBER',            
                Alias = 'alias',          
                TimeZoneSidKey = 'America/Los_Angeles',
                EmailEncodingKey = 'UTF-8',
                LanguageLocaleKey = 'en_US',
                LocaleSidKey = 'en_US'             
            );
            insert u2;

         Group gp = [SELECT ID FROM Group WHERE Name='PCard XYZ CCP Group'];   
        GroupMember gpm = new GroupMember();
        gpm.GroupId = gp.Id;
        gpm.UserOrGroupId = u1.Id;            
        Database.insert(gpm);

        Contact applicant1 = new Contact(firstName='test', lastName = 'applicant11',accountId=acc.id, email ='test.applicant11@abctest.com');
        insert applicant1;
        Contact supervisor1 = new Contact(firstName='test', lastName = 'supervisor12',accountId=acc.id, email ='test.supervisor12@abctest.com');
        insert supervisor1;  
        Contact ao1 = new Contact(firstName='test', lastName = 'ao11',accountId=acc.id, email ='test.ao11@abctest.com');
        insert ao1;              
        
        Card__c app1 = new Card__c(); 
        
        app1.RecordTypeId = pcardRTypeId;
        app1.Cardholder_Name__c  = 'Test Applicant11';
        app1.Cardholder_Email_Address__c='Test.Applicant11@abctest.com';
        app1.PCard_Applicant__c = applicant1.Id;
        app1.PCard_Supervisor__c = supervisor1.Id;
        app1.Approving_Official_Lookup__c = ao1.Id;      
        insert app1; 
            
        Purchase_Card__c app2 = new Purchase_Card__c(); 
        
        app2.RecordTypeId = finRTypeId;
        app2.Cardholder_Name__c  = 'Test Applicant11';
        app2.Cardholder_Email_Address__c='Test.Applicant11@abctest.com';
        app2.Card_Applicant__c = applicant1.Id;
        app2.Card_Supervisor__c = supervisor1.Id;
        app2.Approving_Official_Lookup__c = ao1.Id;
        insert app2; 
   
          
            // Insert Salesforce Files using Apex Class:
         //Create Document
        ContentVersion cv = new ContentVersion();
        cv.Title = 'XYZ Test Document';
        cv.PathOnClient = 'TestDocument.pdf';
        cv.VersionData = Blob.valueOf('Test Content');
        cv.IsMajorVersion = true;
        Insert cv;
                            
             //Get Content Documents
    Id conDocId = [SELECT ContentDocumentId FROM ContentVersion WHERE Id =:cv.Id].ContentDocumentId;
 
    //Create ContentDocumentLink 
    ContentDocumentLink cdl = New ContentDocumentLink();
    cdl.LinkedEntityId = app1.Id;
    cdl.ContentDocumentId = conDocId;
    cdl.shareType = 'V';
    cdl.Visibility = 'AllUsers';
    Insert cdl; 
            
    ContentDocumentLink cdl2 = New ContentDocumentLink();
    cdl2.LinkedEntityId = app2.Id;
    cdl2.ContentDocumentId = conDocId;
    cdl2.shareType = 'V';
    cdl2.Visibility = 'AllUsers';
    Insert cdl2; 
                       
        
         Integer emailbefore = Limits.getEmailInvocations();

         List<String> recordIds=new List<String>{app1.Id};  
         List<String> finRecordIds=new List<String>{app2.Id};  
         XYZLetter.sendLettXYZer(recordIds);   
         XYZLetter.sendXYZLetter(finRecordIds);    
  
        system.assertNotEquals(emailbefore,Limits.getEmailInvocations(),'should have decreased');
        Test.stopTest();         
        
    }
}

}