Unable to resolve dependency for ':app@debug/compileClasspath': Could not resolve com.android.support:appcompat-v7:26.1.0

Unable to resolve dependency for ':app@debug/compileClasspath': Could not resolve com.android.support:appcompat-v7:26.1.0.

Could not resolve com.android.support:appcompat-v7:26.1.0.

Required by: project :app

No cached version of com.android.support:appcompat-v7:26.1.0 available for offline mode.

Error log:

Could not resolve all files for configuration ':app:debugCompileClasspath'.
Could not resolve com.android.support:appcompat-v7:26.1.0.
Required by:
project :app
No cached version of com.android.support:appcompat-v7:26.1.0 available for offline mode.
No cached version of com.android.support:appcompat-v7:26.1.0 available for offline mode.
Could not resolve com.android.support.constraint:constraint-layout:1.1.0-beta3.
Required by:
project :app
No cached version of com.android.support.constraint:constraint-layout:1.1.0-beta3 available for offline mode.
No cached version of com.android.support.constraint:constraint-layout:1.1.0-beta3 available for offline mode.
Could not resolve com.android.support:design:26.1.0.
Required by:
project :app
No cached version of com.android.support:design:26.1.0 available for offline mode.
No cached version of com.android.support:design:26.1.0 available for offline mode.
Could not resolve com.android.support:cardview-v7:26.1.0.
Required by:
project :app
No cached version of com.android.support:cardview-v7:26.1.0 available for offline mode.
No cached version of com.android.support:cardview-v7:26.1.0 available for offline mode.
Could not resolve com.google.android.gms:play-services-ads:11.4.2.
Required by:
project :app
No cached version of com.google.android.gms:play-services-ads:11.4.2 available for offline mode.
No cached version of com.google.android.gms:play-services-ads:11.4.2 available for offline mode.
Could not resolve com.android.support:support-v4:26.1.0.
Required by:
project :app
No cached version of com.android.support:support-v4:26.1.0 available for offline mode.
No cached version of com.android.support:support-v4:26.1.0 available for offline mode.

Error log image:

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

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.

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.

Invalidate Cache / Restart from File option.

Just unchecking offline mode did not work for me.

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.