Issue
When attempting to open macOS Mail, the following error message is seen:
The specified object was not found in the store., The process failed to get the correct properties.
Cause
The macOS Mail database has become corrupted. Other symptoms include missing or garbled text in email messages or missing email.
Resolution
The solution is to rebuild the macOS Mail mailbox. The procedure for this will depend on the version of macOS you are running.
The following Apple article provides instructions for all supported versions of macOS. Select your version of macOS from the top of this document to see the proper procedure:
Rebuild mailboxes in Mail on Mac (Apple Support)