This error means the ULN entered is not in a valid format for a ULN. Ensure you have entered the correct value, or use the PICS - LRS integration to find the correct ULN for the learner.
ESFA Error definition:
The Unique learner number (Learner.ULN), must pass the checksum calculation
(error where DD01 = 'N' or (DD01 <> 'Y' and DD01 <> 10th digit of Learner.ULN))