My employer ask me to resign because of my.medical condition. Is it ok to file a Letter of resignation or i.judt let them terminate my services.

I was operated [spine surgery] last Ocober 13, 2 2020 and out of my work since then. My attending physician told me that i cant go back yet to my work because my job requires a lot of lifting thst might affect my condition. My employer told me that i need to file a letter resignation because they need to fill up my position as soon as possible or they will just terminate me. I want to go back to my work but prevented me becsuse of my.medical condition. Currently, the state of New Jersey is paying me for my temporary disability until end of February 2020 and applying now for an extension. We are an immigrant for 2 years now. Need your help. Thank you and God bless.