点击这个图标来同步gradle

或者编辑你的gradle模块，然后同步

这对我很管用。

文件->设置-> Android SDK -> Android SDK位置编辑-> Android SDK

点击这个图标来同步gradle

或者编辑你的gradle模块，然后同步

修复了3.1.2或更新版本

我和我的一些同事在AS 3.1.2上也遇到过同样的问题，但是一个简单的同步并没有帮助我们。对我们来说，解决方案有点不同:

File -> Invalidate cache -> Invalidate File ->关闭项目。 从AS项目选择器窗口中删除该项目。 退出Android Studio 启动AS并再次打开项目

如果你经常面临这个问题，额外的建议

你们中的一些人经常面临这个问题，而这个“解决方案”只能暂时解决它。在我们开始出现这个错误的时候，我们使用gradle包装器4.6和Android gradle插件3.1.0，但从那以后我们回到gradle 4.4，我们使用了最新的Android gradle插件，从那以后我们就没有看到这个问题了。

进入File >> ProjectStructure(⌘;)。这将打开这个窗口，然后选择应用程序模块。然后选择Compile SDK版本。选择最新的一个，然后单击apply。

After opening my project in Android Studio, I opened the App version of the build.gradle file, located in the Gradle Scripts folder of the Project. When I hovered my mouse pointer over the buildToolsVersion number, a tooltip popped, indicating that the version of Android Build Tools needed to be upgraded (see screenshot). I changed the value of the buildToolsVersion to the one identified by the error (27.0.2) and the problem was solved. The original error (Please select Android SDK) went away and the project compiled successfully, again. Although this solution was mentioned previously, the exact steps seemed to be missing, so hopefully this clears up any doubt. I guess the root cause for this problem is that a new version of the build tools were released but Android Studio did not trigger any notification to me, which would have indicated that I should manually fix this (since Android Studio does not seeem to have the ability to upgrade this build.gradle file internally), which leaves the onus on the user to find the problem and fix it.