There are three possible reasons why you cannot post.
You have reached your limit of posts per calendar day. Users with fame up to 1000 can publish up to 10 posts per day, and with fame over 1000 – up to 20 posts.
You are blocked on the board where you are trying to post. Probably, you have repeatedly violated the board rules, and the moderators added you to the blacklist of this board. If you think that this happened by mistake – you can write to the board moderators in the comments or make a post on the b/moderators board.
You received a temporary ban on the platform. You probably violated the platform rules. If you can still log into your account, then you just have to wait until the ban disappears.
In any case, when publishing a post, you will see a corresponding error. In case of technical problems, you can write at email@example.com.