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.

Work for me 1文件>关闭投影。2打开一个现有的Android Studio项目

按“Ctrl+shift+A”并在弹出的EditText中写入“Sync Project with Gradle Files”。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。

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。

构建开放。gradle文件。只需添加一个空格或按enter键。然后同步项目。它的工作原理……

这是解决这个问题最简单的方法:

在gradle文件中添加注释行 然后它会要求你同步。 点击“立即同步”

中提琴!你就完成了。