I spent a long time on this yesterday after migrating from Android Studio 2.3 to 3.1. Tried everything from other answers to similar questions. Google's maven repository was correctly configured, tried invalidating caches and restarting, restarted the computer, etc.
The one thing that worked for me was closing Android Studio, deleting the .idea
folder from my root folder and starting Android Studio again. Since I had everything checked into git before starting the upgrade I was able to do:
rm -rf .idea/
git checkout .idea
So I ended up with my original idea project files, but you should be fine if you can't revert to your checked in idea files. After reopening Android Studio it recreated some of the files I ended up deleting and everything worked again. Hope that helps others with the same problem.
This answer is old but basically suggests the same thing so I guess this problem keeps showing up. https://stackoverflow.com/a/21100896/791560