_fk_user & " " & get_Year
as auto-entered text should be something you can set with the unique values validation. Are you sure that is what you tried?
Are you sure that this shouldn't include the course ID as well as the user id and year?
It will allow the user to attempt to create a 2nd record with the same ID and year, but you get an error message when you commit the record. There are interface designs you can use that make it impossible to select the same pairing of user and year in the first place.
You might find this demo file helpful in exploring some approaches that manage that: http://www.4shared.com/file/dZ0bjclw/ManyToManywDemoWExtras.html
Thanks for advice,
i select the option "Validate only if field has been modified" on Specify Calculation, now is working.