(Your domain is not yet pointing to our nameservers ns1.infinityfree.com and ns2.infinityfree.com. Please change your nameservers at your domain name provider and wait a few hours for the change to take effect. You can also create an account with a subdomain first.)
The error message is the full answer. Your domain must be pointing to ns1.infinityfree.com and ns2.infintyfree.com to create a new account. If it isn’t, even if the domain is pointing to the same nameservers but with different names, you cannot add the domain.
This also goes for adding domains to existing accounts if they have an if0 username.
But, if you have an account with an epiz_ username, you cannot use the infinityfree.com nameservers, and must use epizy.com nameservers instead.
Alternatively, you can use ns1.byet.org and ns2.byet.org instead. Those nameservers always work.