Pls Help Me With This!--- Android Studio : <!-- What are you trying to accomplish? (Please include sample data.) -->
Can someone help me fix the error seen in the editor ? I'm new to Android Studio and would love to see a fully explained answer.

<!-- Paste the part of the code that shows the problem. (Please indent 4 spaces.) -->Error:Unable to resolve dependency for ':app@debug/compileClasspath': Could not resolve com.android.support:appcompat-v7:26.1.0.
<a href="openFile:C:/Users/MAHE/AndroidStudioProjects/Dork_1/app/build.gradle">Open File</a><br><a href="Unable to resolve dependency for &#39;:app@debug/compileClasspath&#39;: Could not resolve com.android.support:appcompat-v7:26.1.0.">Show Details</a>


Pls Help me fix this

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I got an error![enter image description here][1]


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These are the images...Pls help me!! I am using **Android Studio 3.0**
 

Error Code---
Error:FAILURE: Build failed with an exception.

* What went wrong:
Task '~offline' not found in root project 'Dork_1'.

* Try:
Run gradle tasks to get a list of available tasks. Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org

BUILD FAILED in 2s
