A question that came into my Inbox recently was: can you reverse a PO in R12? I didn't know, so I decided to search out an answer to the question and I found it in the My Oracle Support article "R12: How Do You Reverse PO Matched, Invoice Distributions?" in note 729985.1 which was directly on point. The note said that when you receive an error message of "APP-SQLAP-10103 You cannot reverse this distribution." it is by design as with R12 you cannot reverse matched PO distributions.
What can you do? Well, the note goes on to direct us to go to the Invoice Lines tab and hit the Discard button which with automatically reject the associated PO distributions. To stay up to date with these types of issues, you can join the My Oracle Support community for Payables by clicking this link.