Retrofit is unable to create call adapter

Since you have included addCallAdapterFactory(RxJavaCallAdapterFactory.create()), you are looking to use Observable's to manage your calls. In your interface, explicitly give the parameterized Observable instead of a Call --

@GET(Constants.Api.URL_LOGIN)
    Observable<String> loginUser(@Field("email") String email, @Field("password") String pass, @Field("secret") String secret, @Field("device_id") String deviceid, @Field("pub_key") String pubkey, @Field("device_name") String devicename);

and then your service methods create observables for you that you can subscribe to or use as the start of an observable pipeline.

Observable<String> status = service.loginUser(loginedt.getText().toString(), passwordedt.getText().toString(), secret, device_id, pub_key, device_name);
status.subscribe(/* onNext, onError, onComplete handlers */);