Proof Failed to Generate

If the PDF file isn't locked or containing other add-ons, it might be best to contact Customer Support to troubleshoot the error as they would be able to log in to your instance to see.

Also, sometimes even just circling back later in the day has rectified issues like this for me in the past (momentary glitches).

The error message you attached seems pretty helpful - does the 2nd bullet of it apply to this PDF?
Is it over the size limit? More on this here.

Or it's strange but I've had luck when this happens just refreshing or trying again in a few minutes if I know the file is supported and I shouldn't be getting the error.

Was looking at other known issues and came across this one, good idea for a workaround on here: Upload the document first and then convert to a proof: https://experienceleague.adobe.com/docs/workfront-known-issues/issues/new-workfront-experience/wf-current/wf-proofs-cannot-generate-proof.html?lang=en

I have 'troubleshot" (is that a word?) many, many files that wouldn't render.

In my case, it has always been some wonky element on the page(s) that was causing the problem. I would first try to print it. If that works, and it's a multi-page document, try making proofs of each page individually. That way you'll at least find the offending page. If you find one or more of that pages that won't render, try removing some elements from the page until it renders. You can narrow it down to the offending page elements that way.

Good luck, this can be a tedious process.