If WoW Forever won't create your character, read the exact message on screen. Each one has a different cause. Most players hit one of four: the faction is closed on that ruleset for now, creation is throttled, the name is taken, or the PvP ruleset only allows one faction. Find yours below.

Match your message

What you see What it means Go to
"This faction is currently unavailable on this ruleset due to population imbalance" Blizzard's balance system has paused new characters for that faction on that ruleset Faction unavailable
"Creation of that type of character is currently throttled" That kind of character is limited for now. Blizzard hasn't explained it Throttled
The name is taken or not allowed Someone has that full name, or it breaks the naming rules Name problems
You can't pick the other faction on PvP The PvP ruleset allows one faction per account One faction on PvP
A race or class is grayed out That race can't be that class in Forever Race and class

Faction unavailable

This is the most common one, mostly reported for Horde on the PvP rulesets. It isn't a bug or a problem with your account. Blizzard's ruleset post describes a system that keeps the factions closer in number by pausing one for a while.

What you can do: keep trying at different times of day, or play on Normal, which allows both factions. The full guide is can't create Horde.

Throttled

"Throttled" is a different message from the faction lock. Blizzard hasn't said what triggers it or how long it lasts. Players report getting through by waiting or trying another race or ruleset. See creation throttled.

Name problems

Forever names have a first and a second part, and only the full name has to be unique. If your name is taken, change the second part. Names are region-wide, so a name used anywhere in your region counts. Blizzard explains the rules in its name post. More in name reservation.

One faction on PvP

On the PvP ruleset you can only have characters of one faction. If you already have an Alliance character there, you can't make a Horde one, and the other way round. Normal, Roleplay and Hardcore allow both. See choosing a ruleset.

Race and class

Not every race can play every class, although Forever added new combinations. The races and classes chart shows every allowed pair.

Not one of these