It is not possible to export PNGs with no alpha channel from Figma, unfortunately.
But fret not. It is an easy fix with an ImageMagick subcommand - mogrify
All you got to do is run:
mogrify -alpha off *.png
to turn off the alpha channel for all PNGs in a certain folder.
Enjoy a hussle free upload to App Store with just one command line.
Note, I have checked Explore Opaque plugin, but it never quite worked. It’s really cumbersome and has some bugs, I suppose. It was MUCH, MUCH easier and more straightforward to just export the images and then just run the above command.
If you don’t have ImageMagick installed, it is easy to install with Homebrew.
brew update && brew install imagemagick
Good Luck.