Hi All,

i need to Send an Email When Opportunity stage changes. for this i have Write one Class and Trigger. when i Tried to Insert or Update an Opportunity i have received below error

Error: Invalid Data. 
Review all error messages below to correct your data.
Apex trigger SendEmailToAccount caused an unexpected exception, contact your administrator: SendEmailToAccount: execution of AfterInsert caused by: System.EmailException: SendEmail failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Missing targetObjectId with template: []: Class.HelperContactTrigger.sendEmail: line 34, column 1

Apex Class:
public with sharing class HelperContactTrigger {
    public static List<Opportunity> sendEmail(List<Opportunity>Opportunities)
    {
     //query on template object
        EmailTemplate et=[Select id from EmailTemplate where name=:'Approved By Manager'];

        //list of emails
        List<Messaging.SingleEmailMessage> emails = new List<Messaging.SingleEmailMessage>();   
        
        for(Opportunity Opp : Opportunities)
        {
          //check for Account
            if(Opp.StageName =='Prospecting'|| Opp.StageName=='Qualification'){

                //initiallize messaging method
                Messaging.SingleEmailMessage singleMail = new Messaging.SingleEmailMessage();

                //set object Id
                  List<String> sendTo = new List<String>();
                  sendTo.add('putluruvishnu@gmail.com');
                  singlemail.setToAddresses(sendTo);

                //set template Id
                singleMail.setTemplateId(et.Id);

                //flag to false to stop inserting activity history
                singleMail.setSaveAsActivity(false);

                //add mail
                emails.add(singleMail);
            }
        }
            //send mail
        Messaging.sendEmail(emails);

        return Opportunities;          
        
    }
    
Apex Trigger:
----------------------
trigger SendEmailToAccount on Opportunity (after insert,after update) 
{
    if(Trigger.isAfter)
    {
        if(Trigger.isInsert )
        { 
            //helper class for single email but bulk messages
            HelperContactTrigger.sendEmail(trigger.new);
        }
    }
        if(trigger.isAfter && trigger.isUpdate )
        {           
         HelperContactTrigger.sendEmail(trigger.new);
        }
}