If you're xp locked on a trial/vet account (as in having paid behsten/slahtz to turn off your xp gains), that prevents you from queuing for pvp. There's also a bug that makes it so you can't remove that status on a level 20 toon without paying or GM assistance.
Being unable to gain xp because you're a 20 on an unpaid account but not being xp locked lets you queue for pvp just fine.