Could you help me??

I created the followin trigger which sends e-mail when RFT_Category__c is updated.

But when I try to update it, the error occurrs.

"Apex trigger PDSendEmailtoOC caused an unexpected exception, contact your administrator: PDSendEmailtoOC: execution of AfterUpdate caused by: System.EmailException: SendEmail failed. First exception on row 0; first error: INVALID_ID_FIELD, WhatId is not available for sending emails to UserIds.: []: Trigger.PDSendEmailtoOC: line 26, column 1"

trigger PDSendEmailtoOC on RFT_Category__c (after update) {

  List<RFT_Category__c > pdList= [SELECT Location__c,Object_No__c FROM RFT_Category__c ];
  List<Object_Champion__c> ocList=[SELECT Location__c,Object_No__c,Champion_email__c FROM Object_Champion__c];
  
  for (RFT_Category__c rft : Trigger.new) {
    
    String [] locationList = rft.Location__c.split(';');
    Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
    List<String> toAddresses= new List<String>();
    
    //Set recipients
    for(integer i=0; i<locationList.size(); i++){
      for( Object_Champion__c oc :ocList){
        if( oc.Location__c == locationList[i] && oc.Object_No__c ==rft.Object_No__c ){
          toAddresses.add (oc.Champion_email__c );
        }
      }
    }
    mail.setToAddresses(toAddresses);
    mail.setSenderDisplayName('RFT Auto Mail');
    mail.setTemplateId('00yyyyyyyyyyy');
    mail.setWhatId(rft.Id);
    mail.saveAsActivity = false;      
    mail.setTargetObjectId('00xxxxxxxxxxx');
    Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
  }        
}

 How can I solve this error?

Thanks in advance for your support!!!

Anna