title: Cannot resolve symbol v7 in importing appcompatactivity
body: <p>I'm new to Android Studio and it was working just fine and I made few apps and they worked pretty well, and then I downloaded a project from GitHub and tried to import it, the project didn't work but now every time I make new project this error happens:(I can't upload pictures)</p>
  
  <p>Import <code>android.support.v7.app.AppCompatActivity</code>;
  The 'v7' is colored in red and it says it can't resolve symbol v7</p>
  
  <p>In the messages it giving me this:</p>
  
  <blockquote>
    <p>Unable to resolve dependency for ':app@debug/compileClasspath': could not resolve com.android.support:appcompat-v7:26.0.0-beta1)</p>
  </blockquote>
  
  <p>I tried the following:</p>
  
  <ol>
  <li>invalidate caches and restart</li>
  <li>compile 'com.android.support:appcompat-v7:26.0.0' in the
  dependencies in the build.gradle</li>
  <li>clean project and sync project with Gradle files</li>
  </ol>
  
  <p>But nothing worked.</p>
  
  <p>I have Android studio 3.0.1<br>
  Android emulator 27.1.12<br>
  Android SDK platform tools 27.0.1<br>
  Android SDK tools 26.1.1   </p>