Run pod --version first — if it’s below 1.11.0, that’s your actual problem, not Flutter.
Update CocoaPods with sudo gem install cocoapods (or via Homebrew if you went that route originally).
If the update itself fails or hangs, it’s almost always a Ruby version conflict introduced by the macOS upgrade, not a CocoaPods bug. source

This error usually means Flutter cannot find the latest iOS dependencies. A simple solution is to update CocoaPods and then refresh your Flutter project's iOS dependencies. If the problem continues, remove the old iOS dependency files and let Flutter recreate them. This forces Flutter and CocoaPods to download the latest compatible versions.