@efish88 INCREDIBLE, this SPM issue is present across a ton of forum posts and you have the only solution that worked for me.
For anyone else searching - if you get the error
"when building for watchOS simulator, no library for this platform was found"
or "when building for visionOS simulator, no library for this platform was found"
For any Swift Package (in my case, FirebaseAnalytics) you can solve it by deleting any existing scheme for the platform (watchOS & visionOS in my case), creating a new scheme for the platform, and as stated uncheck everything on the main app. It should look like this
After completing those changes, just select the correct scheme before attempting to preview the product (in this case, I've selected my watchOS scheme) and the preview should work smoothly