Try these commands:

flutter clean
rm -rf ios/Podfile.lock ios/Pods/
flutter run

// This worked for me most of the time. I believe the consequence 
// of a Podfile older than Podfile.lock is that pod install isn't 
// executed as part of flutter run. The above fixes that.

// Another thing to try if the latter fails, check all the packages. One of them might not support Flutter 2 yet 
// or vice versa.For this go to your files and see if an of the imports have warnings.
// Tricky error lots of ways to solve the error but more info would help.