Could not find any version that matches com.android.support:appcompat-v7:27.+

在仓库中加入google()，或者maven { url 'https://maven.google.com' }

allprojects {
    repositories {
        jcenter()
        // apparently google() can be used too, but I couldn't get it to work
        // google()
        maven { url 'https://maven.google.com' }
    }
}