QB Issue Resolution:

Odd that it is not working for you, I just tried mine and it works fine

on my PO, I have the inventory items listed in the second section of the PO called item details

then I use create new transaction button>bill from the vendor listing, the bill screen comes up, the PO is listed on the right side and click the add link

would you explain how you are doing it if different

Resolution for Issue 'How do we copy a PO for inventory to a bill?' available: Yes (Solved).
Source: Intuit Community forum.

How do we copy a PO for inventory to a bill?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Inventory
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