Most probable Cause

This issue is most probably because the package name was renamed, but the package name in build.gradle was remained unchanged. To solve, just change the package name in build.gradle.

Other issues
When you rename the package name Android Studio should change the package names in AndroidManifest.xml automatically. In some cases Android Studio does not do this. So to fix the issue, open AndroidManifest.xml and change all occurrences of the old package name with your package name.