There can be 2 reasons:
1. Low level characters don't appear on armory. You can try to add it to friends list in game to check if character exists.
2. WoW client includes a list of names that can not be used because they are against Blizzards character naming policy:
https://us.battle.net/support/en/article/naming-policy
On wotlk realms characters sometimes don't get deleted properly and their names become available only after server restart. That bug might exist on TBC realm also so it's third possible reason but it's much less likely than the other 2.
That's disabled on all realms except Blackrock as far as I know.