If someone is not setup as an approver for the PO Module, the below rejection will occur in the workflow
You would need to make sure the user is setup with the right PO Roles:
1. Setup the user in User ID Cross-Reference
2. Assign the Purchase Order Roles, If they should be a Requestor / Buyer or Receiver:
3. If they are supposed to be an approver, you need to add that here: