Full Error Message:
Error while sending email. Error message: Send Email failed. First exception on row 0; first error: INVALID_ID_FIELD. Invalid entity of whatID.
-You may receive this error message when trying to generate a document with email feature in document action and there may be issue with your Salesforce settings so that it will not be able to send.
How to resolve:
Check that you have the “Allow Activities” checkbox checked on the standard or custom object being used as base object in document action. Once this is checked, you should be able to send an email without error.