Rash is a good thing. It is indicative of immunotherapy efficacy. I am hoping to develop rash soon for this reason. I am three weeks into nivolumab (opdivo) myself. I have had iraes (side effects) so far: fatigue, back/shoulder pain (intermittent), diarrhea (intermittent), abdominal pain (intermittent).
https://www.mesothelioma.com/blog/signs-immunotherapy-may-be-working-for-you/#:~:text=Itchy%20skin%20and%20rashes%20are,Bladder%20cancer