Customer records in Unleashed cannot be used as a Supplier record for a Purchase Order. Similarly, Supplier records cannot be used as a Customer in Sales Orders. Customers and Suppliers in Unleashed are separate record types and cannot be interchanged. 

If you have Customer that you also purchase stock from, you will need to create a new Supplier record for them to create a Purchase Order in Unleashed with their details. For more details on creating a Supplier, see Add Supplier