Workflow Error : method RELEASE_PURCH_DOC cannot be executed

I am getting following errors in  single Workflow for PURCHASE ORDER RELEASE

1. User 01101685 already processing Purchase order 30 00938139

2. Work item 000017292981: Object PO#/User method RELEASE_PURCH_DOC cannot be executed

Inside the Method RELEASE_PURCH_DOC function module

'BAPI_PO_RELEASE' is called.

and I have checked in container values , I have found that PO Release code is not updated from BAPI_PO_RELEASE.

Please help on this.

Thanks,

Arpita