Solution For An Error : Resolve this dependency for ':app@debug/compileClasspath': Could not resolve com.android.support.appcompact.

• Your computer must be connected to internet to resolve this problem.

• Android Studio > File >  Settings > Build, Execution, Deployment

•  Gradle

• Uncheck the offline work and click OK to exit from Settings window

•  Go to Build Menu > Rebuild project

• Click the Try again link in Gradle notification and your pc will download the required library files from the internet and your problem will be resolved.