Syncing Android Studio project with Gradle files

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?

Solution 1:

EDIT

Starting with Android Studio 3.1, you should go to:

File -> Sync Project with Gradle Files

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

Solution 2:

Old Answer

When trying to run the application, instead of selecting the directory highlighted here in blue

I selected the subdirectory instead

and clicked "run".All the issues with Gradle are automatically resolved and the missing apk directory is automatically created.

New Solution

The Sync project with gradle files button disappeared from Android Studio for a while.Its back and you can find it here:

hit the button and wait for the task to complete

Solution 3:

I've had this problem after installing the genymotion (another android amulator) plugin. A closer inspection reveled that gradle needs SDK tools version 19.1.0 in order to run (I had 19.0.3 previously).

To fix it, I had to edit build.gradle and under android I changed to: buildToolsVersion 19.1.0

Then I had to rebuild again, and the error was gone.