我去建造。Gradle和点击同步现在。然后就成功了。

更新:

文件->同步项目与Gradle文件(Android Studio 3.1.1)

工具-> Android ->同步项目与Gradle文件(Android Studio 3.0.1)

或者您可以单击工具栏上的图标。

这个答案可能对以后的版本没有帮助，因为Android studio团队正在努力使工具变得更好，在下一个版本的Android studio中，同步的方式可能会有所不同。

常见的方法，可能有助于尝试同步项目，然后无效缓存和重新启动Android工作室。

Android Studio 3.1.2解决方案[见下文答案]

请参阅最新的Android Studio版本

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。

这对我很管用。

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

有很多答案，我尝试使用其中一个解决方案，在构建中添加新的行。Gradle文件，但它没有工作。然后我将comileSdkVersion更改为另一个版本，然后将其更改为我拥有的版本。“同步”按钮出现。按下它，为我解决了问题。

转到:文件>项目结构>模块->应用>属性

请选择:编译“SDK”版本= API26构建工具 版本号= 27.0.0源兼容性= 1.8目标兼容性= 1．8

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.