Is there a way to restrict use of certain values in a lookup field in PPM under certain circumstances (example not allow users to change the status of a project to Closed unless the project status is Approved)?
One option would be to use parameterized lookups that can set up dependencies where certain values in one attribute are only available if the criteria is met in the related attribute. See the below related documentation for more details on creating Parameterized Lookups:
Generating a custom error message (including messages based on certain value selections) is unfortunately not available in CA PPM (Project & Portfolio Management) out of the box. The one option would be to create a hint for the field, but this would display all the time, not based on certain criteria.
To add a hint: