as依赖解决报错

Unable to resolve dependency for ':app@debug/compileClasspath': Could not resolve com.android.support:appcompat-v7:27.0.2.
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Unable to resolve dependency for ':app@debugAndroidTest/compileClasspath': Could not resolve com.android.support.test:runner:1.0.1.
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Unable to resolve dependency for ':app@debugAndroidTest/compileClasspath': Could not resolve com.android.support.test.espresso:espresso-core:3.0.1.
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Unable to resolve dependency for ':app@debugAndroidTest/compileClasspath': Could not resolve com.android.support:appcompat-v7:27.0.2.
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Unable to resolve dependency for ':app@debugUnitTest/compileClasspath': Could not resolve com.android.support:appcompat-v7:27.0.2.
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Unable to resolve dependency for ':app@debugUnitTest/compileClasspath': Could not resolve junit:junit:4.12.
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Unable to resolve dependency for ':app@release/compileClasspath': Could not resolve com.android.support:appcompat-v7:27.0.2.
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Unable to resolve dependency for ':app@releaseUnitTest/compileClasspath': Could not resolve com.android.support:appcompat-v7:27.0.2.
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Unable to resolve dependency for ':app@releaseUnitTest/compileClasspath': Could not resolve junit:junit:4.12.
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