androidx.lifecycle..DefaultLifecycleObserver not found

I think that's because you're missing one dependency:

implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"

Get the latest version of lifecycle-common-java8 here

See the documentation:

If you use Java 8 Language, then observe events with DefaultLifecycleObserver. To include it you should add "androidx.lifecycle:lifecycle-common-java8:" to your build.gradle file.