Ultra key effect cannot be exported

you might try using the smart render workflow

https://community.adobe.com/t5/premiere-pro-discussions/faq-what-is-smart-rendering/td-p/10648488

Theoretically if you have your timeline completely rendered using a high quality preview format and "use previews" when you export, you should be good to go...

by any chance are any of your sources from a smartphone or a screen recording?  If so, the issue may be caused by variable frame rate (vfr) sources.  Here's how to diagnose and fix the problem

use mediainfo to determine whether your source is variable or constant frame rate
https://mediaarea.net/en/MediaInfo/Download
if it's variable use handbrake to convert to constant frame rate setting the quality slider in the video panel to maximum
https://handbrake.fr
and here's a tutorial on how to use handbrake
https://www.youtube.com/watch?time_continue=34&v=xlvxgVREX-Y

Try turning off Hardware encoding in the export settings.

Hello ,

I wonder if clearing your preferences and trashing your media cache would help. There may be something left over in those files that isn't supposed to be there.

Preferences: FAQ: How to reset (trash) preferences in Premiere Pro?

Media Cache: FAQ: How to clean (delete or trash) media cache files?

Let me know if this works!