This problem usually occurs when a school has recently converted to an Academy and it does not have URN details entered in SIMS.
Go to Focus>School>School Details
You will need to enter the Unique Reference Number (URN) even if this number has not changed since the School became an Academy.
You must also enter the Date Number Changed (for the URN) again even if this has not changed. In this case the Date will be the Date the School became and Academy.