Mistake at section 9

[orcchg]

It says

Remove the Call wrapper from the return type. Here we're returning String, 
but you could return complex json-backed type as well. If you still wanted 
to provide access to Retrofit's full Result, you can return Result<String> 
instead of String from the suspend function.

but basically there is no type Result<T> in Retrofit. This should be Response<T> instead.