Go to File->Other Settings->Default Settings->Build, Execution, Deployment->Build Tools->Gradle->Uncheck Offline work option.

Just went to build.gradle and deleted the line:

implementation 'com.android.support:appcompat-v7:26.1.0'

After that, I re-synced the Gradle. Then, I pasted the line of code back, re-synced the Gradle again and it worked.

Note: While I was making this changes, I also updated all the SDK Tools that needed update.

Below is a workaround demo image of ; Uncheck Offline work option by going to:

File -> Settings -> Build, Execution, Deployment -> Gradle

If above workaround not works then try this:

1. Open the build.gradle file for your application.

2. Make sure that the repositories section includes a maven section with the "https://maven.google.com" endpoint. For example:

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

3. Add the support library to the dependencies section. For example, to add the v4 core-utils library, add the following lines:

   dependencies {
   ...
   compile "com.android.support:support-core-utils:27.1.0"
   }
   

Caution: Using dynamic dependencies (for example, palette-v7:23.0.+) can cause unexpected version updates and regression incompatibilities. We recommend that you explicitly specify a library version (for example, palette-v7:27.1.0).

Manifest Declaration Changes

Specifically, you should update the android:minSdkVersion element of the <uses-sdk> tag in the manifest to the new, lower version number, as shown below:

<uses-sdk
android:minSdkVersion="14"
android:targetSdkVersion="23" />

If you are using Gradle build files, the minSdkVersion setting in the build file overrides the manifest settings.

apply plugin: 'com.android.application'


android {
...
defaultConfig {
minSdkVersion 16
...
}
...
}

Following Android Developer Library Support.

use Invalidate Caches/Restart it will works for you and make sure that your gradle offline sync option is not enabled

Adding this in project gradle worked for me

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

Invalidate Cache / Restart from File option.

Just unchecking offline mode did not work for me.

Today I have the similar problem.

Previous in my project gradle

// Top-level build file where you can add configuration options common to all
sub-projects/modules.
allprojects {
repositories {
jcenter()
google()
mavenCentral()
maven { url "https://jitpack.io" }
maven { url 'https://plugins.gradle.org/m2/'}
}}

Then I just added this below line in allprojects

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

It saved my day.

And now my current allproject {} code looks like this

allprojects {
repositories {
jcenter()
google()
mavenCentral()
maven { url "https://jitpack.io" }
maven { url 'https://plugins.gradle.org/m2/'}
maven {
url "https://maven.google.com"
}
}}

For users which have flavors in the project and found this thread:

Notice, that if your module dependency has different flavors, you should use one of the strategies:

1. Module that tightens dependencies should have the same flavors and dimensions as the dependency module
2. You should explicitly indicate which configuration you target in the module

Like that:

dependencies {
compile project(path: ':module', configuration:'alphaDebug')
}

I ran into the same issue and adding mavenCentral() in the repositories block of my build.gradle file worked for me. I had Offline work unchecked and adding maven { url "https://maven.google.com" }. Adding this answer for the lost souls that find themselves at end of this thread.

From the documentations:

Add the support library to the dependencies section. For example, to add the v4 core-utils library, add the following lines:

dependencies {
...
implementation "com.android.support:support-core-utils:28.0.0"
}

I solved the problem. In Home Ubuntu, I deleted the .gradle folder and downloaded it again. I hope it is useful

If you did not uncheck offline work, then make you sure you have internet for Android Studio to normalize your project.

If you are getting this same error after adding dynamic module then don't worry follow this:

• Add productFlavors in your build.gradle(dynamic- module)

  productFlavors {
  flavorDimensions "default"
  stage {
  // to do
  }
  prod {
  // to do
  
  
  }
  }
  

For other people where the accepted answer does not solve this issue...

As of September 2018, AndroidX has replaced the Android Support Libraries, which includes the appcompat-v7 library.

'com.android.support:appcompat-v7' becomes 'androidx.appcompat:appcompat:1.0.0'

'com.android.support:design' becomes 'com.google.android.material:material:1.0.0'

References:

https://developer.android.com/jetpack/androidx/migrate List of Support Libraries to AndroidX mappings

This is a problem for users who live in a country that is banned by Google (like Iran). for this reason we need to remove these restrictions by a proxy. follow me :

file->settings->Appearance&Behavior->System Setting-> Http Proxy-> Manual proxy configuration ->HTTP -> Host name : fodev.org ->Port : 8118 .

and click Ok Button. then go to file-> Invalidate Caches/Restart . . . Use and enjoy the correct execution without error ;)

Go to File->Other Settings->Preferences for New Projects->Build, Execution, Deployment->Build Tools->Gradle->Uncheck Offline work option.

In the android\app\build.gradle file,

Replace the version details

implementation 'com.android.support:appcompat-v7:26.1.0'

with

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

Android Studio will use a suitable version to make it work for you.

For me the cause was incorrect url for jcenter. To solve it I simply changed url for app and proect level build.gradle

from

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

to

jcenter()

Go to build.Gradle file and replace 27 and 29 by 25 at these places 1. targetSdkVersion 25 2. implementation 'com.android.support:appcompat-v7:25.+'

it really works for me Thanks.

Used implementation 'androidx.appcompat:appcompat:1.1.2' in App gradle fixed the issue for me