Android no longer downloading the libraries from the SDK manager, it has to be accessed through Google's Maven repository.
You will have to do something similar to this in your build.gradle file:
allprojects {
repositories {
jcenter()
maven {
url "https://maven.google.com"
}
}
}
dependencies {
...
compile "com.android.support:support-core-utils:27.0.2"
}
Find more details about the setting up process here and about the different support library revisions here.