For some reason since the new site has been launched this has been a much more common question than it ever was before. The answer to this (and many questions) is in the site rules and FAQ but they are often unread. The end result is to "PM all mods" Just in short to answer this question the reasons that you can't post fall 99% of the time into one of three categories:
1. You haven't activated your account - When you sign up an email is sent to you as verification, in that email is an activation link that you must click before you can post. If you don't see the email check your spam folder and if it isn't there you can have the confirmation email sent again by going here. If you just joined the site and you can't post then this is most likely the reason.
2. You are a new user - New users are limited to a number of posts per day until they get past a certain point which will make them have unlimited access to posting.
3. You karma is too high - You get one karma point any time you flag or get flagged. If you ask the mods why you can't post and you have either been flagging a bunch of people, or posting stuff that you know will probably get flagged then this is your answer.
4. You have been banned from the site - This is a rarer case, and if you were banned you should have gotten a notification from a mod or staff explaining why it was done. And you should know why you were banned as the mods or staff will have given you previous warnings.