All you need to do is to find a folder called "flutter" (lowecase), which is located inside a folder called "Flutter" (uppercase), select it, and browse it.
In my case it is located at:
C:\Users\Administrator\Flutter\flutter_windows_v1.12.13+hotfix.5-stable\flutter
Also make sure that your Flutter and Dart are of the latest version. If they are not, upgrade them and re-start the IDE.