PO's can be created with existing suppliers only that have been setup under the Suppliers section in administration.
If you would like to have this option enabled so users cannot add new suppliers into the system and only create PO's with existing suppliers then the admin will need to go to Admin > Suppliers
Once in the suppliers section check the option for 'Create P.O.s with existing suppliers only.' like shown below:
If this option is selected and a new supplier needs to be created the admin will need to create the supplier by going to Admin > Suppliers > Add New Supplier.