Run Button is Disabled in Android Studio

Click Run on the menu and then Edit Configurations... then click on Android Application on the left and click the + button. Choose Android Application from the pop-up menu. Then pick the module (its normally app or something like that). Then click apply and ok.

If you have more errors after that, try to re-import the project in Android Studio.

When creating the Run Configuration, the dropdown for Module had only <no module> for me. Invoking menu File -> Sync Project with Gradle Files added app to the dropdown for Module. Then the Run button became enabled.