When using a full setup, you can’t change the NS records for the root of you domain. Those will be automatically set to your assigned nameservers. You can, however, delegate a child zone if you need to. Skip the next link for now. It will still be here later if want to read it. You will want to read the part after it first.
In order to activate your domain in Cloudflare, you need to change the nameserver records at your domain registrar. That appears to be Synergy Wholesale Accreditations Pty Ltd, or more likely, a reseller partner. You can read more about this part of the setup in the following guide.
If you have more questions about the process, let us know.