Error:(39, 13) Failed to resolve: com.android.support:appcompat-v7:26.0.0 [duplicate]

Answer 1

Add this to your project level build.gradle file:

repositories {
    maven {
        url "https://maven.google.com"
    }
}

Answer 2

UPDATE: In Android studio 3.0 the error is solved.
Or you can add google() to solve it since you will use new the gradle plugin as mentioned here.

buildscript {
    repositories {
        ...
        // You need to add the following repository to download the
        // new plugin.
        google()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.0'
    }
}

---

Before Android studio 3.0: Check if the support library is installed in your system (check the SDK directory in your system) or check if Android O is installed in the SDK Manager.

Or as this answer stated

Starting from version 26 of support libraries make sure that the repositories section includes a maven section with the "https://maven.google.com" endpoint.

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