Proving self-employment has stopped for Universal Credit claim

I have been told I need to prove that my self-employment has ended in order for my Universal Credit claim to be processed. I reported the change to HMRC 6 weeks ago and still no letter to confirm my status. I also tried to file a tax return but the website won't allow me to file for 24/25 yet. What can I do?

Hi
You can find out when you can expect to receive a reply form HMRC here:  
Check when you can expect a reply from HMRC
Thank you