我的问题是，如果我改变了项目(例如用不同的项目打开Android Studio的新窗口)，就会发生这种错误。

所以我阅读了这个问题的每个答案，“清除缓存并重新打开项目”的解决方案在我的情况下并不是那么有用。

相反，我发现如果Kotlin插件在Android Studio中的版本低于gradle文件中的版本，问题就会出现。

我的设置是:

Android Studio 3.1.3 Kotlin插件(在Android Studio) 1.2.30 Kotlin版本在gradle文件1.2.51

是什么解决了这个问题?

我更新了Kotlin插件:

文件->设置->插件->安装JetBrains插件…

然后搜索Kotlin并单击Update。更新后只需重新启动Android Studio和同步Gradle。

它帮助了我:

1. close project
2. remove .idea/ folder
3. select "import project (Gradle, Eclipse ADT, etc.)"

环境:

Android Studio 3.1.3
OS Macos High Sierra 10.13.5

另一个答案不适合我。

我的解决方案 这个虫子快把我逼疯了。再多的同步也无法修复它。我发现如果我修改我的模块的构建。在AndroidStudio中添加gradle(可能会在行后添加一个空行或空格)，然后同步该文件，一切都很好。

只需将构建工具版本更改为25.0.3..现在开始同步 我希望它会有所帮助。

在Mac上按“⌘+Shift+A”(在Windows上按“Ctrl+Shift+A”)，并在弹出的EditText中写入“将项目与Gradle文件同步”。然后双击出现的选项。然后它将同步您的Gradle文件SDK与项目文件。

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.