Purchase order requires supplying site

I'm trying to create a purchase order from line items but it keeps saying I need to select a Supplying Site before I can save. I should add the following:
the line item I wish to create a PO for is an item I have created which is a 'purchased' item. It is classed as a 'sold' item e.g. we buy in the items for resale.

I can't see what this means at all since the line items are already created? Any help much appreciated.