If you know the ID you want to delete, you don't need to involve Table1
DELETE FROM TABLE2 WHERE Req_ID = 2If you want to select the qualifying rows and delete just one of those, use this:
;WITH cte AS
SELECT TOP (1) * FROM dbo.Table2 t2
WHERE NOT EXISTS
SELECT 1 FROM dbo.Table1 t1 WHERE t1.Req_Id = t2.Req_id
) DELETE FROM cte;
I am assuming that logically there should be a foreign key relationship, but there is no such constraint in the database. If there was, this situation should not have come about in the first place.