Kindly take a backup before you proceed further. SQL backup or server snapshot or trimmed backup whichever is faster.
Step 1: Delete all the requests in your ServiceDeskPlus MSP application.
Note: Also, ensure to delete all requests from the TRASH as well.
Step 2: Connect to the database and execute the query,
delete from seqgenstate where seqname like ‘workorder%’;
Step 3: Restart ManageEngine ServiceDeskPlus MSP service.