QB Issue Resolution:

bring up the PO on screen, top left just under the vendor name is an orange kind of looking thing, use the drop down arrow, then use the next drop down arrow again and select closed

Resolution for Issue 'How do I close a Purchase order that has already been completed and paid for, but is still showing up as open underneath the vendor it was purchased from?' available: Yes (Solved).
Source: Intuit Community forum.

How do I close a Purchase order that has already been completed and paid for, but is still showing up as open underneath the vendor it was purchased from?: 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 completed
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