Purchase Orders can only be matched to Bills if:
Purchase Order and Bill have the same Vendor (which determines the Currency of both documents)
The Purchase Order is Approved and not yet Closed, during Bill creation and up until final Bill approval
The Purchase Order date is not later than the Bill date (retrospective Purchase Orders are not allowed to be matched in QuickBooks Online)
The Product/Service or Category is the same in the matched lines of the Purchase Order and Bill
The lines have not yet been matched