I had the same thing happen to me using ArcPro 3.1.3 . It is very frustrating. I tried a bit of trouble shooting including trying to Print to PDF or Cute PDF, clearing the cache, etc. I didn't want to re-install Pro, as I read that it doesn't always work. When I created a new project and brought over my files and created a new layout the Export to PDF worked the first time around!
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