Hi Nidhi,
First have a cross check whether you could get the PO related workflows in SBWP(SAP Inbox) to identify the custom workflow is working file.
If yes, maintain your workflow task ID under below path in SPRO
Materials Management->Purchasing->Purchase Order->Approve purchase order App->Specify workflow task IDs for Approve PO app
Once this is done, you will have to maintain event linkage and agent assignment in transaction code "OOCU" under the path MM->MM-PUR
regards,
Sabarinathan C