So what's the problem?
Looks like the system is giving you a perfect guidance what to do.
It's got nothing to do with the work centre directly, but the IT0001 record you are updating with this assignment has a non-existing value in the contract field. Add it to the table for the respective countrry or change field content.
Or does your post mean this didn't work? Then please let us know what happened when you tried to follow the instructions.