Manger needs to know medical reason

Not sure if this is the right place to post but I got a doctors note from my family doctor that said I can’t work mids(Friday,Saturday) while in college and instead of putting a specific reason on the note it said due to medical reasons. I was told they can’t accept the doctors note because they need to know what the medical reason is. Is this allowed? Other information -I live in Ontario -I work part time at McDonald’s