With the update of 3.3. Guild Wars is only playable every third week. If you get the message that you are unable to register for Guild Wars, this is because the Guild Wars event is not running.
You will be able to play Guild Wars every three weeks as it now rotates with Raid Bosses and Invasions every other week. Raid Bosses and Invasions allow you to work with your Guild in exciting new ways to defeat NPCs and unlock new rewards such as Orbs. For more information on Raid Bosses please visit the Update 3.3 Patch Notes here.