Hi NovaConcepts
Welcome to the Community Forum. To explain why it is happening, first I'll explain the difference between the type of transactions.
A purchase order is a purchase where no service/item has been received yet.
A bill is a purchase where either the service/item are received or payment has been requested by the supplier.
As purchase orders are usually estimations and not the final payment required, generally you would not need to link a document to that type of transaction, so the In Tray is designed to store documents against bills only.
For this reason it is only possible to link a document to a bill. If you choose Purchase Order after clicking Create New Bill, it is no longer a bill and therefore no document will be linked. If you have purchase orders that have a document you would like to attach, you will need to convert it to a Bill first.
Hope that helps.
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