Unable to create call adapter for io.reactivex.Observable

You need to tell Retrofit that you want to use RxJava 2, using:

addCallAdapterFactory(RxJava2CallAdapterFactory.create())

So, for creating your Retrofit object, you will have something like:

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl(SERVICE_ENDPOINT)
    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
    .build();