Purchase requisition (PR) status is Submitted and the purchase order (PO) not issued. 

When checking the approval flow, each approver has taken action and approved the PR, however there is one approval node in status Pending - No Approver Selected. 

Delete the node, and the PO will be generated.

This can occur if a user has tried to manually add an approver, but before saving their changes, they exited the browser.

SAP Ariba Buying registers an approver was attempted to be added, but because the user failed to properly save the approval flow change, a Pending - No Approver Selected node gets added to the flow. 

Core Procurement > Core Administration > Approval Flows
Purchasing
Purchasing > Purchase Orders