Please ensure that the guild you are trying to join is not close for postulates. Or check if you have already postulated on another guild. This could stop you from applying to different guild. To see if you have, please check what is written under your status (veteran) on the left hand side of the screen. If a guild name is shown there, you can either cancel your postulate on that guild or wait for them to accept you.
If you didn't apply to another guild and yet you still can't postulate to other guilds, please contact me (or any other moderator) or go to the customers' support services and tell them about the problem.