Android Gradle Sync failed: Could not resolve all artifacts for configuration ':classpath'

It is a problem found in the new Android Studio 3.6 update. But it is basically not a problem.

All you have to do is the following:
To enable or disable Gradle's offline mode, first select View > Tool Windows > Gradle from the menu bar. Then, near the top of the Gradle window, click Toggle Offline Mode

Here are some screenshots:

1. First go here
   
2. Toggle this option

I was going through similar issues last night, this is what worked for me :

Open the build.gradle file from your app folder, it should look similar to this

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.6.1'
    }
}

allprojects {
    repositories {
        jcenter()
    }
}

add google() to repositories in -buildscript and -allprojects, just like here :

buildscript {
    repositories {
        google()       // here
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.6.1'
    }
}

allprojects {
    repositories {
        google()      //and here
        jcenter()
    }
}

In Android Studio, go to File -> Project Structure.. [ shortcut : Ctrl + Alt + Shift + S ]

In the Project tab -> Android Gradle Plugin Version -> 3.6.1 and Gradle Version -> 5.6.4 |||

Last, under the Module tab, make sure the compile sdk version is > 14 (ie: 25, 28 or 29..)

solution is to enable gradle as highlighted in below picture:

I opened an old project with Eclipse. For me, it helped to add the following in build.gradle:

apply plugin: 'com.android.application'

repositories {
    // order may be important!
    google()
    jcenter()
} 

Source: https://stackoverflow.com/a/50285646/1097104

In addition, a few times I had to use File > Invalidate Caches / Restart.

If you are using Android Latest version Chipmunk. Then this solution for you.

step 1. Open Gradle Setting and copy gradlePluginPortal() and paste it in dependencyResolutionManagement below.

step 2. Change RepositoriesMode.PREFER_SETTINGS like this step 3. enable android.enableJetifier=true

follow the above step then definetelly solve your issue.

The project that you are trying to build probably doesn't have the required SDK version. You can download it from the Android Studio within the "Preferences" dialog, under Appearance & Behavior → System Settings → Android SDK. Preferences Screenshot.