Error adding alias -- This domain name is not registered

I am using Cloudflare for my DNS, and I added a subdomain cname that points to my InfinityFree account and when I put it in on add alias, I get the following error message.

This domain name is not registered, we can register this domain name for you

when I try to add it.

When I do a nslookup on the sub-domain, this is what I get:

nslookup sheddesigner1.3dfish.net

Server: cdns01.comcast.net
Address: 2001:558:feed::1

Non-authoritative answer:
Name: sheddesigner1.rf.gd
Address: 185.27.134.222
Aliases: sheddesigner1.3dfish.net

@SirLogic said:
I am using Cloudflare for my DNS, and I added a subdomain cname that points to my InfinityFree account and when I put it in on add alias, I get the following error message.

This domain name is not registered, we can register this domain name for you

when I try to add it.

When I do a nslookup on the sub-domain, this is what I get:

nslookup sheddesigner1.3dfish.net

Server: cdns01.comcast.net
Address: 2001:558:feed::1

I don’t think so IPv6 is provided by Infinity Free for free hosting

Non-authoritative answer:
Name: sheddesigner1.rf.gd
Address: 185.27.134.222
Aliases: sheddesigner1.3dfish.net

Domains must be registered with a valid registrar before they can be parked. In addition, they will not be functional unless they are configured to point to these DNS servers:
ns1.epizy.com
ns2.epizy.com

To add a domain to your account, it MUST point to our nameservers. After adding the domain you can switch to your own nameservers again, but for verification you must be able to change the nameservers of your domain.