Batch class which is not working :

global class exportobjectascsvbatch implements Database.Batchable <SObject>{
global Database.QueryLocator start (Database.BatchableContext bc)
{
    String SobjectApiName = 'Opportunity';
Map<String, Schema.SObjectType> schemaMap = Schema.getGlobalDescribe();
Map<String, Schema.SObjectField> fieldMap = schemaMap.get(SobjectApiName).getDescribe().fields.getMap();
String query = 'Select ';
for(String fieldName : fieldMap.keyset() )
    {
        query += fieldname+',';   
        }

query = query.removeEnd(',');
query += ' from Opportunity where lastmodifieddate = today limit 200';
    return Database.getQueryLocator(query);
}
   global void execute(Database.BatchableContext bc, List<Opportunity> OppList)
   {
       try{     
String SobjectApiName = 'Opportunity';
Map<String, Schema.SObjectType> schemaMap = Schema.getGlobalDescribe();
Map<String, Schema.SObjectField> fieldMap = schemaMap.get(SobjectApiName).getDescribe().fields.getMap();
String generatedCSVFile = '';
for(String fieldName : fieldMap.keyset() )
    {
        generatedCSVFile += fieldName +',';     
        }
generatedCSVFile = generatedCSVFile.removeEnd(',');
generatedCSVFile = generatedCSVFile + '\n'; 
for(Opportunity company: OppList){
  for(String fieldName : fieldMap.keyset() )
    {
        generatedCSVFile += company.get(fieldName)+ ',';     
        }
    generatedCSVFile = generatedCSVFile.removeEnd(',');
    generatedCSVFile = generatedCSVFile + '\n'; 
}
//send email with generated csv file
Messaging.EmailFileAttachment csvAttachment = new Messaging.EmailFileAttachment();
Blob csvBlob = blob.valueOf(generatedCSVFile);
String csvName = 'Opportunitybackup.csv';
csvAttachment.setFileName(csvName);
csvAttachment.setBody(csvBlob);
Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
String[] toAddresses = new String[]{'thanigai.k.balaji@apisero.com'};
String subject = 'Opportunity backup';
email.setSubject(subject);
email.setToAddresses(toAddresses);
email.setPlainTextBody('Opportunity backup');
email.setFileAttachments(new Messaging.EmailFileAttachment[]{csvAttachment});
Messaging.SendEmailResult[] r = Messaging.sendEmail(new Messaging.SingleEmailMessage[]{email});
       }
       catch(exception e)
       {
            system.debug('Exception Caught:'+e.getmessage());
       }
       }
       
    global void finish(Database.BatchableContext bc)
    {
    }  
}




Apex class which works as expected:

public class exportopp { 

    public static void exportopp() {    
List<Opportunity> extractOpplist = new List<Opportunity>();

   List<Opportunity> OpportunityList = new List<Opportunity>();
        
String SobjectApiName = 'Opportunity';
Map<String, Schema.SObjectType> schemaMap = Schema.getGlobalDescribe();
Map<String, Schema.SObjectField> fieldMap = schemaMap.get(SobjectApiName).getDescribe().fields.getMap();
String query = 'Select ';
for(String fieldName : fieldMap.keyset() )
    {
        
        query += fieldname+',';
        
        }

query = query.removeEnd(',');
        
        query += ' from Opportunity where lastmodifieddate = yesterday';
       
system.debug(query);
List<Opportunity>  OppList = database.query(query);

system.debug(opplist);


String generatedCSVFile = '';
for(String fieldName : fieldMap.keyset() )
    {
        generatedCSVFile += fieldName +',';     
        }
generatedCSVFile = generatedCSVFile.removeEnd(',');
    generatedCSVFile = generatedCSVFile + '\n'; 


for(Opportunity company: OppList){
  
for(String fieldName : fieldMap.keyset() )
    {
        generatedCSVFile += company.get(fieldName)+ ',';     
        }
    generatedCSVFile = generatedCSVFile.removeEnd(',');
    generatedCSVFile = generatedCSVFile + '\n'; 
}
    
Messaging.EmailFileAttachment csvAttachment = new Messaging.EmailFileAttachment();
Blob csvBlob = blob.valueOf(generatedCSVFile);
String csvName = 'Opportunity backup.csv';
csvAttachment.setFileName(csvName);
csvAttachment.setBody(csvBlob);
Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
String[] toAddresses = new String[]{'thanigai.k.balaji@apisero.com'};
String subject = 'Opportunity backup';
email.setSubject(subject);
email.setToAddresses(toAddresses);
email.setPlainTextBody('Opportunity backup');
email.setFileAttachments(new Messaging.EmailFileAttachment[]{csvAttachment});
Messaging.SendEmailResult[] r = Messaging.sendEmail(new Messaging.SingleEmailMessage[]{email});
}
}


can anyone able to check what causing the issue and also if possible please share test class for apex class