Need to validate custom field based on Assignment Category both in Employment Details section.

Summary:

Hi experts,
we are trying to build an autocomplete rule which validates a custom field (pink) based on the Assignment Category by giving an error message if its populated.
It should trigger only when a certain value is chosen in Assignment Category. They are both in the same section - Assignment Details
Is it possible with autocomplete rule to create this validation?
We have tried to use Object Validation and Field Validation rule type but both triggers incorrectly, as the error message shows only after inserting for the second time the value.

Please let us know.

Thank you

Version (include the version you are using, if applicable):