Failed to resolve: com.android.support:appcompat-v7:27.+ (Dependency Error) [duplicate]

Find root build.gradle file and add google maven repo inside allprojects tag

repositories {
        mavenLocal()
        mavenCentral()
        maven {                                  // <-- Add this
            url 'https://maven.google.com/' 
            name 'Google'
        }
    } 

It's better to use specific version instead of variable version

compile 'com.android.support:appcompat-v7:27.0.0'

If you're using Android Plugin for Gradle 3.0.0 or latter version

repositories {
      mavenLocal()
      mavenCentral()
      google()        //---> Add this
} 

and inject dependency in this way :

implementation 'com.android.support:appcompat-v7:27.0.0'

If you are using Android Studio 3.0 or above make sure your project build.gradle should have content similar to-

buildscript {                 
    repositories {
        google()
        jcenter()
    }
    dependencies {            
        classpath 'com.android.tools.build:gradle:3.0.1'

    }
}

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

Note- position really matters add google() before jcenter()

And for below Android Studio 3.0 and starting from support libraries 26.+ your project build.gradle must look like this-

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

check these links below for more details-

1- Building Android Apps

2- Add Build Dependencies

3- Configure Your Build