Could not find com.android.support:support-fragment:27.1.1

Studio 2.3版本应该在Root Project build.gradle allprojects配置如下：

allprojects {
    repositories {
        jcenter()
        maven { url "https://maven.google.com" }//添加
    }
}

Studio 3.0版本应该在Root Project build.gradle allprojects配置如下：

allprojects {
    repositories {
        jcenter()
        google()//添加
    }
}