The reason for this error is that the Receipt generated from confirming the Purchase Order has a conflicting number with an existing transfer. The solution to this problem is to update the Sequences to start from the next available number. 

First activate developer mode using the link at the bottom of Settings. When back on the Settings page, go to Technical menu -> Sequences. Here you will see a sequence that is assigned to the operation type that gives the error with a perticular prefix. Click on this sequence and click on Edit, and change the Next Number to the next available number. Then click Save.

After performing these steps you should be able to confirm the Purchase order. If you have any incoming receipts before you get this message, you may need to check the next number in the sequence yourself by going to Inventory -> Operations menu -> Transfers, searching for your prefix and sort by Reference descending. You will see the last number for your prefix at the top of the list and you can enter the number after the last number as the Next Number in the Sequence as mentioned above.