Presently, Purchase Orders created manually in QuickBooks Online or via ApprovalMax seem to differ a bit. For POs created in ApprovalMax, you won't see the "Copy to Bill' button in the dropdown menu on the right and will not see them in the reporting tool in QuickBooks Online either.
We are currently trying to figure out why that is and how it can be fixed.
Meanwhile, there is a simple workaround: you just need to open the PO in QuickBooks Online after it was published there by ApprovalMax and click the "Save" button. It then immediately gets the same status as a PO created directly in QuickBooks Online.