This will relate to the linked Entity for the destination in which that year's Activity Survey is being run. You can clear this error by following the steps below;
Select the YP's Basic Details page.
Click Destinations.
Click the Destination that is relevant for that Activity Survey period.
Click on the green arrow next to the Current School.
Enter a 7-digit Establishment number. If this is an unknown destination, you will need to set this as a default value in the format of your 3-digit LEA code followed by four 9s, e.g. 1119999.