My employer denying to issue me my relieving letter

Hi sir, Could you please suggest me what should I do, my previous employer is denying me relieving letter stating " your separation is marked as unauthorised absentism". I recovered from COVID back in August 2022, since then I had severe breathing issues and severe chest infection. I continued job till Oct 3, 2022. Then took 5days leaves but my situation worsened as my hands and upper chest getting numb. I informed about the same to my team leader. I formally gave my resignation by email and sent it to my team leader which he asked me to send it, then TL forwarded the resignation email to my manager, later told me to handover computer as I'm working from home, which I did return on oct 16, 2022. I informed my tl about the same and he told me to wait for HR communication which I didn't get. After 60 days I sent email to HR, they sent they can't give me reliving letter. Later for next 6months I sent the same email and received same response. I've fibromyalgia and I've been unemployed for over a year. Im taking care of my health and Now im trying to get another job, which I can't without relieving letter. I don't want to go with fake experience letters for my next job. Please help. Regards Mohammad Yusuf