尝试构建应用程序时，每次与Gradle同步时，都会出现相同的3个错误

有人说，错误:配置项目':app'时出现问题.

one that says, Error:A problem occurred configuring project ':app'.

Could not resolve all dependencies for configuration ':app:_debugCompile'.
   Could not find com.android.support:appcompat-v7:21.1.2.
     Searched in the following locations:
         https://jcenter.bintray.com/com/android/support/appcompat-v7/21.1.2/appcompat-v7-21.1.2.pom
         https://jcenter.bintray.com/com/android/support/appcompat-v7/21.1.2/appcompat-v7-21.1.2.jar

     file:/C:/Users/Richard/AppData/Local/Android/sdk/extras/android/m2repository/com/android/support/appcompat-v7/21.1.2/appcompat-v7-21.1.2.pom
     file:/C:/Users/Richard/AppData/Local/Android/sdk/extras/android/m2repository/com/android/support/appcompat-v7/21.1.2/appcompat-v7-21.1.2.jar
     file:/C:/Users/Richard/AppData/Local/Android/sdk/extras/google/m2repository/com/android/support/appcompat-v7/21.1.2/appcompat-v7-21.1.2.pom
     file:/C:/Users/Richard/AppData/Local/Android/sdk/extras/google/m2repository/com/android/support/appcompat-v7/21.1.2/appcompat-v7-21.1.2.jar
 Required by:
     MyApplication:app:unspecified

另外一个说:

Error:(28, 13) Failed to resolve: com.android.support:appcompat-v7:21.1.2
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