While running npx cap run android if you are facing the following error:
native-run failed with error ERR_SDK_NOT_FOUND: No valid Android SDK root found. More details for this error may be available online: https://github.com/ionic-team/native-run/wiki/Android-Errors
The solution is to install Android Studio https://developer.android.com/studio.
Installing the Android Studio will automatically install the Android SDK needed for the project.