Syncing Android Studio project with Gradle files

Question

I was working on a project, and then I got a prompt to update Android Studio. After I did that, I started getting this error when trying to run my app

It says

The project may need to be synced with Gradle files

How do I solve this?

Answer 1

EDIT

Starting with Android Studio 3.1, you should go to:

File -> Sync Project with Gradle Files

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OLD

Clicking the button 'Sync Project With Gradle Files' should do the trick:

Tools -> Android -> Sync Project with Gradle Files

If that fails, try running 'Rebuild project':

Build -> Rebuild Project