Hi,
Are you getting this error first time in your system? If am not wrong, you should have faced this problem earlier also. If not, can you check if any new changes have been moved to PRD.
The error screenshot says that the assignment number (which you can see Del to Bil copy controls) is being used while releasing the billing to accounts. Check any of the existing accounting document and see how the assignment number is taking.
Regards, Sai Krishna.