I have it working using PDF. I had it configured correctly to use Keynote. When trying to use PPTX or KEY file from OpenLP I get the error mentioned in my original post. I created a text Keynote file to see if it was the process I used converting PPTX to Keynote. That gave the same error.
I wrote a C# app to loop through my 990 PPTX files and convert them to PDF. I transferred those to the Mac and they work. I tested with 1 PDF before converting to make sure it worked.
Actually I guess I got the information from the FAQ: PowerPoint, Keynote and LibreOffice are not supported on macOS due to limitations with the platform. This is outside of our control.
If that is true I assume it is due to the inability to use interoperability?