This seems to be the problem when you use OkHttp along with Retrofit, so either you can remove OkHttp or use code below to get error body:
if (!response.isSuccessful()) {
InputStream i = response.errorBody().byteStream();
BufferedReader r = new BufferedReader(new InputStreamReader(i));
StringBuilder errorResult = new StringBuilder();
String line;
try {
while ((line = r.readLine()) != null) {
errorResult.append(line).append('\n');
}
} catch (IOException e) {
e.printStackTrace();
}
}