I know about this questions:
Failed to resolve: com.android.support:cardview-v7:26.0.0 android
Could not resolve com.android.support:appcompat-v7:26.1.0 [dup
I tried all of the above and nothing worked for me.
Then I followed Gradle Settings > Build Execution, Deployment > Gradle > Android Studio and checked "Disable embedded Maven repository".
Did a build with this checked and the problem was solved.