RxAndroid和Retrofit:无法为io.reactivex.Observable>创建调用适配器

I am trying use rxJava, rxAndroid, Retrofit2, and OkHTTP3 to download a file from a URL endpoint. My code is unable to create the call adapter for an "Observable< retrofit2.Response< okhttp3.ResponseBody>>". These methods are new to me so I believe I'm missing an important concept here. Any direction or points is greatly appreciated.I am trying use rxJava, rxAndroid, Retrofit2, a