Hi Nate,
Thanks for following up!
Unfortunately, Apple’s documentation has all been updated for iOS 26 which isn’t a great deal of help when iOS 18 is still the publicly available version 
, but you can find the previous version on the Wayback Machine at least.
It’s been a while since I personally added anything to the App Store, but IIRC, in Xcode, you can provide assets for the various icon styles (Light Mode, Dark Mode, Tinted). I believe for Tinted, if you don’t provide one yourself, Apple does some magic to create a reasonable variant. We posted an article about creating icons for iOS 18 which also may provide some tips.
For these icons I believe transparency is supported, at least for the tinted variant.
You mentioned that you received the error when uploading it to Apple so I’m thinking that you were uploading the icon through App Store Connect? In that case, the icons uploaded there are used for the App Store rather than the Application itself so these would need you to include a background of some sort (ideally matching the default light mode iOS icon).
If that’s not the case, and if you’re still having problems, if you can clarify a little about where exactly you’re uploading it to then that would certainly help!