I am receiving this error message:
Step Not yet complete... here's what's wrong: 
An update to a Battle Station record with Weapon Status of 'Fully Operational' failed to post a Chatter message to the record. Make sure that the process is configured correctly and that it is activated. 
Note: you may run into errors if you've skipped previous steps.

The Validation rule is correct, the "Project Status' field update occurs correctly, the post is made to the chatter feed of the record (mine and the SF record that is used to verify the step) - and yet I get the error.
How should I troubleshoot this?