COVID-19, like other viruses, is best fought with a strong immune system. In fact, the stronger the better.
If we get our hands on an effective vaccine, the benefit of it will be giving our immune systems the tools to combat the illness.
Immunosuppresants work to weaken the immune system. They benefit a very specific group of people with very specific medical conditions, but will make things substantially worse for anyone trying to fight a viral infection like coronavirus.
If you have a specific concern about a specific medication, be sure to speak with a medical doctor, but as a general question, that info, I hope, is helpful.