You can’t change the nameservers of a domain registered with Cloudflare Registrar, as is indicated at the time of purchase. You should be able to point at your hosting using A/AAAA/CNAME records, see “How To Connect a Domain By Adding A-Type Records” down the page here…
If you still want to change the nameservers, you will have to transfer the domain to another registrar after the 60-day transfer lock period.