Hello Stevenson, usually this error is related to lack of completion or passing the wrong object as "Data object" input in the "SendEmailWithTemplates" java action.
Check if the object you are passing has been configured correctly within the template page where you create the tokens to be used in the email.
For more information you can consult the official documentation:
https://docs.mendix.com/appstore/modules/email-connector
or this video on YouTube from "MendixWithMe" channel:
https://youtu.be/3Pn2c7rm5t4
If you are using the Email Connector module, check that you are using the SendEmailWithTemplate action to send your email. This should replace the tokens in the EmailTemplate object with values from the Data Object.
More details can be found in the documentation.
https://docs.mendix.com/appstore/modules/email-connector/#433-sending-an-email-with-a-template
Good luck!