The domain step offers four options, and choosing the wrong one stalls the order.
REGISTER A NEW DOMAIN
Search and add it. Registered in your name, billed separately from hosting on its own annual cycle.
TRANSFER A DOMAIN TO US
Moves an existing registration here. Needs the EPP code, the domain unlocked, and the domain more than 60 days old. Usually adds a year.
USE A DOMAIN I ALREADY OWN
Choose this when the domain stays registered elsewhere and you want hosting only. Nothing is billed for the domain, and you point its nameservers at us afterwards.
USE A SUBDOMAIN
Where offered, gives a temporary address for testing. Not for a real business site.
THE MOST COMMON MISTAKE
Choosing "register a new domain" for a domain you already own. The registration fails and the order stalls. If you already own it, choose the third option.
BEFORE CONFIRMING
Check the spelling character by character. Domain registrations are not refundable, and a typo becomes a domain you own and do not want.