In our case it was because we were using the ad-hoc provisioning profile rather than app-store. We've always done this but something seems to have changed (maybe xcode 12, maybe something on the store side). Switching to app-store made it work, though the .ipa to be uploaded through transporter is roughly 10 times the size (not sure why, I hope the app size as apparent to users isn't similarly bloated).

Having this issue now in React Native project having upgraded to RN 0.63.3 and Xcode to 12.1. Not sure what to do....