Use Validation at the time Creating Purchase Order

Hi Guru

W create a Validation rule for certain GL Account associates with Cost centers. When I am crating a document is FB50, it create mssage "Must Post Account XXXXXXXXXX to XX-XX tier Cost Center. but at the time of creating purchase order in Account Assignemnt table I allow to use any combination of Gl account and cost center. Business requirement is to create a same error message as we are getting at the time of creating document in FB60.

Please advice.