Create purchase requisition - Enter only one true account assignment

Hello all,

I am currently setting up the SAP project system. I now want to create a purchase requisition with account assignment P (Project). I have to ente a GL account number and a WBS element and a network number. Once I save the purchase requisition, I get the message:

Enter only one true account assignment

Message no. KI249

Diagnosis

You made assignments to several objects in CO (cost center, order, project etc.). 2 of these have been created as true objects.

System Response

You are allowed only one account assignment for each cost-relevant account.

Any suggestions?

Thanks

Anne