In regards to being unable to post, it's due to your current in game level for the character you currently have associated to this forum account not being high enough (according to the character info under your avatar it says you're only a level 24 ARC) - there are posting restrictions on this forum based on your character's level in game, level 50 is necessary to have the most unrestricted access to the forum, as stated in this official thread.

If you have another character that is much higher than that, then you need to change the character associated with the forum account, and/or the class/Job associated with that character, which can be done by clicking the 'Settings' link at the top right hand corner of the page (next to the blue 'Logout' button), then selecting 'Edit Character' from the list of options on the left, and then selecting a desired character from the list of characters that are active on that account (the one currently associated will be at the top of the list and listed as 'Your Current Main Character', and in the list of characters, the current one will have a yellow star on it's avatar and the others will be greyed out). To change the character, click on the avatar of the character you wish to switch to, it should now be in colour and have a yellow star on it's avatar.

Additionally, to change that character's class, there are three drop down menu buttons underneath each character's tab, with the left-most class option being the one that will be 'associated' with that character as far as the forum is concerned (and thus dictates your posting restrictions). So it's preferable to associate a class or Job that is at least level 50 or higher to it so you have the least possible restrictions. Then when you're done click 'Save Settings' at the bottom of the page to confirm.

Hopefully this has helped.