Error Message - Reference number of a document from Back Error

Dear Guys,

I got error message "Reference number of a document from Back Error" after creating the PO).

The SAP screen showed as below once I enter the PO.

Reference Number of a Document from Back Error

Item Service                                                                    Status

PO #                             Blocked  / Checked

     Custom document                                                     Blocked / Checked

10 Embargo Check                                                         Released / Checked

10 Sanctioned Party List Screening                                 Blocked / Checked

Please clearify what is the block and what is the source for this error message.

Thanks

Paranthaman