INVALID_ID_FIELD, WhatId is not available for sending emails to UserIds

Introduction

This error is thrown while sending email using apex if you are setting User Id in target recipient: setTargetObjectId(usr.Id) and also specifying target record id as: setWhatId(opp.Id). As per Salesforce documentation if you are passing record id in WhatId then you can only pass Contact Id in setTargetObjectId.

Here is Salesforce article:

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_email_outbound_single.htm

Workaround

• Send to Contacts: If you can send it to contact instead of user then it’s ok to use WhatId along with email template.
• Send to users with custom email body: If you are supposed to send it to Users only then do not set WhatId, instead prepare email body in apex and pass it as below:

Messaging.SingleEmailMessage message = new Messaging.SingleEmailMessage();
message.setTargetObjectId(targetId);//DIRECT USER
message.setSubject('Email from Salesforce');
message.setPlainTextBody('A record has been updated');
message.saveAsActivity = false; 

• Send to users with template: If you are supposed to send it to Users only then do not set WhatId, instead get the email content from template as below and set it as Body as below:

Messaging.SingleEmailMessage message = Messaging.renderStoredEmailTemplate(TEMPLATEID, USERID, RECORDID);
String emailSubject = message.getSubject();
String emailTextBody = message.getPlainTextBody(); 
Messaging.SingleEmailMessage message = new Messaging.SingleEmailMessage();
message.setTargetObjectId(targetId);//DIRECT USER
message.setSubject(subject);
message.setPlainTextBody(emailTextBody);
message.saveAsActivity = false;