MM-PO Error Z1 when saving a modification or creation of a Purchase Order (PO) in SAP

CONTEXT: When you try to save a PO after modification or creation you face this error : 'Following mandatory roles missing in partner maintenance: Z1

RESOLUTION: How to solve it 

1. add manually in the partner tab of the PO : the function Z1
2. copy the supplier number contact to be used

RESULT: You can now proceed with the update or creation of your Purchase order