Hi stf1,
Thanks for your query in this community.
I have performed the test on my side with MacOS Mojave 10.14.4 & PowerPoint 16.24 190414. Unfortunately, I could not reproduce this issue on my side. Please check my image below:
I noticed that in your screenshot, you have 28 slides in this presentation, and you selected print All slides, but the under the preview panel, it's still 1 of 1. In this kind of scenario, to help us narrow down the issue, please check my following suggestions:
1.Click on the Print, will the Notes be exported to PDF without issue?
2.In the print Layout, could you switch to the other options, and will the preview appear?
3.Create a new presentation, and check if the preview panel is still blank out.
Regards,
Alex Chen