IllegalArgumentException: Could not locate call adapter for rx.Observable RxJava, Retrofit2

Be sure to add implementation 'com.squareup.retrofit2:adapter-rxjava2:2.4.0' or whatever version you are using to your dependencies, and then configure retrofit with that converter:

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl(endpoint)
    .addConverterFactory(GsonConverterFactory.create())
    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
    .build();

Updated

RxJavaCallAdapterFactory was renamed to RxJava2CallAdapterFactory. Changed the snipped above.