Is my employer allowed to force me to give them a written recommendation from my doctor in order to continue to stay home but still be employed?

I am high risk for contracting the covid19. My job allowed me to stay home without pay, because I am high risk. I am still employed. My supervisor told me I could stay out until July 31st. Now they are informing me that I need a written note from my doctors office that they reccomend I stay out due to my underlying conditions. My doctors office refuses to write anyone recommendations, regardless of their high risk status. I have a written text asking how long I was allowed to stay out, and a response from my supervisor that I was allowed to be out until July 31st. If I dont give my employer the letter within the week, I will be required to go back to work Monday, or they'll write that I voluntarily quit. I dont want to lose my job, and I dont want to lose my life. I feel like I'm being wronged, and I'd appreciate some clarification.