Error when syncing project unkwnown property '$androidxVersion'

[phillat]

Please I get this error when syncing my project with the dependency. The error is given below.

Gradle sync failed: Could not get unknown property 'androidxVersion' for object of type org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.

Update:
Fixed the issue by addding the specific version number of androidx.preference, that is, implementation "androidx.preference:preference:1.0.0". But still wondering wasn't the version number provided by $androidxVersion?

[7200rpm]

That is just an arbitrary variable for holding the latest version info so one doesn't need to keep updating documentation. You can just put in the actual version as you stated.