Purchase Order not created from requisition automatically

Summary:

We have a backing BPA, but PO is not being created from requisition automatically. In 'Process Requisition' page, it shows warning: 'The requisition line wasn't processed into a purchase order because a buyer couldn't be determined.'

Content (required):

We have BPA created with 'Automatically generate orders' and 'Automatically submit for approval' enabled. We created a requisition and entered BPA details in it while creating it. It was approved. But it did not automatically create PO. It was stuck in 'Process Requisition' page. It shows warning: 'The requisition line wasn't processed into a purchase order because a buyer couldn't be determined.' We could manually process it to create a PO, but why did it not create a PO automatically? We are using same user for all this testing, it is set up as a procurement agent and has roles