Hi @Farhan_Abbas,
I think you are missing a user entry for a corresponding review assignment. Try this query:
SELECT ra.review_id, ra.reviewer_id AS user_id FROM review_assignments ra LEFT JOIN users u ON (ra.reviewer_id = u.user_id) WHERE u.user_id IS NULL;
If this provides any results, it means you are missing a user entry with the corresponding user_id.
Regards,
Alec Smecher
Public Knowledge Project Team