Could not resolve all dependencies for configuration ':classpath'

Right, I'm not sure if it will work for others but worked for me.

I changed proxyPort to 8080 and used jcenter instead of Maven. But I had to apply expeption to use HTTP instead of HTTPS. This is what I have in my build.gradle for build script and allprojects

buildscript {
  repositories {
    jcenter {
        url "http://jcenter.bintray.com/"
    }
  }
}

allprojects {
repositories {
    jcenter {
        url "http://jcenter.bintray.com/"
    }
  }
}

UPDATE: 06/08

I have recently updated Gradle and plugin version and had some problems. It was complaining about plugin com.android.application

I did some digging around and changed

 jcenter {
    url "http://jcenter.bintray.com/"
}

to

repositories {
    maven { url 'http://repo1.maven.org/maven2' }
}

Find and Replace:

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

to:

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

For newer android studio 3.0.0 and gradle update, this needed to be included in project level build.gradle file for android Gradle build tools and related dependencies since Google moved to its own maven repository.

// Top-level build file where you can add configuration options common to all sub-projects/modules.

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

        // NOTE: Do not place your application dependencies here; they belong
    }
}

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

task clean(type: Delete) {
    delete rootProject.buildDir
}

I'm using Gradle plugin 3.0.1 and saw this error. Not sure what caused this but the solution that works for me is to stop the running Gradle daemon by ./gradlew --stop.

late but it worked for me:
change your dns setting by going Network and sharing center.
In left pane choose change adapter setting
-right click on your network connection
-properties
-select ipv4
-properties
- now in dns server setting:-
-choose the option use the following dns server addresses
and use google dns server 8.8.8.8 as preferred dns server and 8.8.4.4 as alternate dns server.

it will solve your problem.