Hi,
Try this query:
SELECT T3.[U_NAME] FROM OPOR T0 inner join OWDD T1 on t0.docentry = t1.docentry inner join WDD1 T2 on T1.[WddCode] = T2.[WddCode] INNER JOIN OUSR T3 ON T2.UserID = T3.USERID WHERE T2.[Status] = 'y' and T1.[ObjType] = 46 and t0.docnum = $[OVPM.docnum]
Also refer thread Show Approver Name on Purchase Order
Thanks & Regards,
Nagarajan