Cannot setup floating action buttons as class cannot be found (Android Studio)

Question

So I am pretty new to android and I am trying to setup a floating action button with the Android Design Support Library using this guide in Android Studio.

My Projec

Answer 1

I have been facing this problem recently and tried to change dependencies version, invalidating caches and restart, etc. The preview rendered once and then reopening my xml file, it raised the same error.

Clicking on the refresh button at the preview window solved to me (momentarily) the problem, letting me see the layout and its elements properly.