Melatonin is a hormone that affects a lot of different body functions, including strengthening the immune system. We're trying to suppress our overactive immune response with PMA.

The Mount Sinai website points out the benefits of Melatonin, but says:
"Steroids and immunosuppressant medications: Melatonin may cause these medication to lose their effectiveness. DO NOT take melatonin with corticosteroids or other medications used to suppress the immune system."
https://www.mountsinai.org/health-library/supplement/melatonin

In Australia, melatonin is available only on prescription in 2mg tabs. There is an exception for over 55yo who may buy one month supply from a chemist after discussing their other meds and needs. If you're on steroids you're directed to ask your doctor before taking melatonin. I'm surprised how freely available it is in some countries.