Check you AndroidManifest.xml has the correct name in the Activity
Another option:
File -> Invalidate Caches / Restart...
http://stackoverflow.com/questions/15825081/error-default-activity-not-found
I couldn't find this:
In Android Studio, right click on the project and choose Open Module Settings. Then go to the**Sources** tab in your module, find the src folder, right click on it and mark it as Sources (blue color).
http://stackoverflow.com/questions/18828654/default-activity-not-found-in-android-studio
Another error that went away with the above suggestions:
Gradle signing app with packageRelease “specified for property 'signingConfig.storeFile' does not exist”
