I assume you have already
ios/ folder in your Flutter project. If not, you can run
flutter build ios.
The root cause of this error most of the times happens after you have cloned a project like from Github.
In order to fix, you need to get the packages and install the pods.
flutter clean && flutter pub get && cd ios/ && pod install
(In some situation you might need to update your Pod Repo and Pods. You can do that via running
pod repo update && pod update)