This is still a problem in the full ios14 release

Errors mentioned in here while deploying to simulator using xcode 12 are also one of the things which have affected me. Just right clicking on each of my projects and showing in finder, opening the .xcodeproj in Atom, then going through the .pbxproj and removing all of the VALIDARCHS settings. This was is what got it working for me. Tried a few of the other suggestions (excluding arm64, Build Active Architecture Only) which seemed to get my build further but ultimately leave me at another error. Having VALIDARCH settings lying around is probably best thing to check for first.