In my case, my app is on react native, and the problem was due to firebase/app, I had to delete the dependency on react native, run npm i, modify Podfile to comment out the references to Firebase related pods that apparently only work if you use modular headers run pod cache clean --all and pod install to remove the dependencies and the error disappeared. The thing it seemd to be, that using those modular headers somehow "brakes" the UIKit, resulting on several errors. For now I was using firebase to distribute, but I'm very disapointed because I wanted to implement push notifications using firebase, and I haven't found anyone that could implement it succesfully on iOS, so I'm investigating other options, as codepush.