android studio 3.1.3 Unable to resolve dependency

android studio 3.1.3 Bulid 一直报错，网上各种方法都无效。报错信息：

Unable to resolve dependency for ':app@debug/compileClasspath': Could not download appcompat-v7.aar (com.android.support:appcompat-v7:27.0.1)

Unable to resolve dependency for ':app@debug/compileClasspath': Could not download support-fragment.aar (com.android.support:support-fragment:27.0.1)