Dart sdk not configured
File->Settings->Language & Framework->Flutter
Choose flutter SDK path: the first time we install flutter, we choose the location where the flutter should be installed. Choose this location.
Click OK and the android studio will refresh. Carry on if the problem is solved.
If you are still stuck with the error.
Goto this link and install Dart.
Goto the same place in settings, ..Language & Framework->Dart and chose the SDK location.