Error:Apex trigger UpdateOpptIndirect caused an unexpected exception, contact your administrator: UpdateOpptIndirect: execution of AfterUpdate caused by: System.EmailException: SendEmail failed. First exception on row 0; first error: INVALID_FIELD_WHEN_USING_TEMPLATE, When a template is specified the plain text body, html body, subject and charset may not be specified : []: Class.AP_OpportunityUpdate.NotifyOpportunityOwner: line 74, column 1

CLASS :-

public class AP_OpportunityUpdate
{
public static boolean blnflag=false;  
public static void upDateOpt(list<Opportunity> lstOpt,Map<id,Opportunity> mpoldvalues)
{
map<id,id> mapOpportunityOwnerId=new map<id,id>();
Set<id> stAccountId=new set<id>();

list<UserRole> userRoleMap=new list<UserRole>([select Id from UserRole where name='indirect']);
blnflag=false;
for(Opportunity opt: lstOpt)
{
    if(opt.ownerid==userinfo.getuserid() && userinfo.getUserRoleId()!=userRoleMap[0].id && opt.StageName!=mpoldvalues.get(opt.id).stageName && opt.StageName=='Closed Won'  )
    {
        mapOpportunityOwnerId.put(opt.id,opt.ownerid);
        stAccountId.add(opt.accountid);
    }
}


set<id> stEmail=new set<id>();
list<Opportunity> lstOpprtunityToUpdate=new list<Opportunity>();
for(Opportunity varOpts:[SELECT AccountId,Owner.name,Owner.email,owner.userRole.Name FROM Opportunity  where AccountId In :stAccountId AND owner.userRole.Name='Indirect'])
{
//stEmail.add(varOpts.Owner.Email);
stEmail.add(varOpts.Ownerid);
  varOpts.StageName='Closed Lost';
    varOpts.Won_Lost_Reason__c='Won by Current Channel';
    lstOpprtunityToUpdate.add(varOpts);

}
update lstOpprtunityToUpdate;
NotifyOpportunityOwner(lstOpprtunityToUpdate);
//NotifyOpportunityOwner(stid);
blnflag=true;
}
public static void NotifyOpportunityOwner(list<opportunity> lstOpprtunityToUpdate)
{
/*list<id> lstmail=new list<id>();
lstmail.addall(stmail);*/

list<Messaging.singleEmailMessage> lstMailmail = new list<Messaging.singleEmailMessage>();

for(opportunity varOpt: lstOpprtunityToUpdate)

{
Messaging.singleEmailMessage mail = new Messaging.singleEmailMessage();
 
//list<string> lstmails =new list<string>();
//lstmail.addall(stmail);
//string strusr1='';
/*for(integer i=0; i<lstmail.size(); i++ )
{
if(i==0)
{
strusr1=lstmail[i];
}
lstmail.remove(strusr1);*/
//mail.setToAddresses(lstmail);

 // The email template ID used for the email
   mail.setTemplateId('00XK0000000DjTU');
 mail.setTargetObjectId(varOpt.ownerid);

  mail.whatid=varOpt.id;
  mail.saveAsActivity = false;
    //mail.whatid='001K000000MqbEp';
 
 lstMailmail.add(mail);  
system.debug('---------------->'+lstMailmail.size()); 
system.debug('---------------->'+lstMailmail); 
if(!lstMailmail.isEmpty())
{   
Messaging.sendEmail(lstMailmail);
}
}
}
}

Trigger :-

trigger UpdateOpptIndirect on Opportunity (After update) 
{
if(AP_OpportunityUpdate.blnflag==false)
{
AP_OpportunityUpdate.upDateOpt(trigger.new,Trigger.oldmap);

}




}

i have written a class which i'm calling through trigger .

I have a account say TEST :-

TEST account has several opportunities in its related list i.e both DIRECT and INDIRECT.

and opportunity are of 2 types direct and indirect based on user's role and record type i.e if user role is indirect then its an indirect opportunities.

So,

I want ..If the account owner has an existing opportunity  of his own and it’s marked closed/won then all the Indirect opportunity stage  related to account TEST should change to as closed/lost with a reason of “won by current channel”