android studio找不到android.support:appcompat-v7:21.0.3？

Question

错误：（25）评估根项目'Side-Menu.Android-master'时出现问题。

  

无法应用插件[id'android-reporting']

     

配置项目'：app'时出现问题。

  > A problem occurred configuring project ':library'.

     > Could not resolve all dependencies for configuration ':library:_debugCompile'.

        > Could not find com.android.support:appcompat-v7:21.0.3.

          Searched in the following locations:

              https://jcenter.bintray.com/com/android/support/appcompat-v7/21.0.3/appcompat-v7-21.0.3.pom

              https://jcenter.bintray.com/com/android/support/appcompat-v7/21.0.3/appcompat-v7-21.0.3.jar

          Required by:

              com.yalantis:library:1.0

第一次使用android studio并遇到这个问题，帮帮我吧！