One thing you can do in Xcode 12 is to test purchases in the simulator, if you set up a StoreKit Configuration file and enable it in the project scheme... Then you can do the full range of purchase testing in the simulator. On device Sandbox should still present a login (make sure your scheme does not have a StoreKit configuration active if you want to test using Sandbox), if you go to Settings->AppStore are you already logged in to a testing account? If you are it would not present a new login. I get the same crash in Xcode using the Transactions window and clicking on "manage subscriptions", though Xcode 12.2 seems to have a release candidate now and that might fix things.

Just wanted to add a bit to this since I came to this thread looking to resolve why my framework builds did not include simulator binaries for both M1 Macs (ARM) and Intel. I was using the same "xcodebuild archive -scheme StructKit -destination "generic/platform=iOS Simulator" ..." command mentioned before, however for me I only ever saw an intel binary in the resigning framework generated. What ended up being the fix, was that inside my .xcproj file some old version of Xcode has left a default "archs" value for simulators that looked like this: VALID_ARCHS[sdk=iphonesimulator] I removed those from two places in the .xcodeproj file, and after that the xcodebuild command produced a framework that had a fat binary with both ARM and Intel.