Could not find any version that matches com.android.support:appcompat-v7:30.+.报错处理方法

Unable to resolve dependency for ':app@debug/compileClasspath': Could not find any version that matches com.android.support:appcompat-v7:30.+.

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dependencies {

    implementation fileTree(dir: 'libs', include: ['*.jar'])

    implementation 'com.android.support:appcompat-v7:30.+'

    implementation 'com.android.support.constraint:constraint-layout:1.1.0'

    testImplementation 'junit:junit:4.12'

    androidTestImplementation 'com.android.support.test:runner:1.0.2'

    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'

}

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改后再重新编译就没问题了，见加粗语句：

dependencies {

    implementation fileTree(dir: 'libs', include: ['*.jar'])

    implementation 'com.android.support:appcompat-v7:+'

    implementation 'com.android.support.constraint:constraint-layout:1.1.0'

    testImplementation 'junit:junit:4.12'

    androidTestImplementation 'com.android.support.test:runner:1.0.2'

    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'

}