As using Cloudflare requires the use of Cloudflare nameservers, you can’t change the nameservers of a domain registered with Cloudflare Registrar as was indicated at the time of registration or transfer.
You either need to use your Cloudflare DNS to point at your host’s A/AAAA/CNAME records, or, if you want to change the nameservers, transfer your domain out to another registrar.