Android --- Unable to resolve dependency for ‘:app@debug/compileClasspath‘: Could not resolve com.a

网上说了一堆要修改gradle.properties文件，注释掉后四行，如果没有这个文件呢？如果没有设置代理呢？你可以常识一下方法

buildscript {
    repositories {
        maven{ url 'http://maven.aliyun.com/nexus/content/groups/public/'}
        maven { url 'https://maven.aliyun.com/repository/google' }
        //jcenter()
        //google()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.4.2'
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        //google()
        //jcenter()
        maven{ url 'http://maven.aliyun.com/nexus/content/groups/public/'}
        maven { url 'https://maven.aliyun.com/repository/google' }
    }
}

注：gradle是你原本的版本就可以