When there then there should be a text box you can type things in. I am not sure if new users have restrictions like they might for links in posts or something. Equally you do have options to variously disable posts entirely, or disable them for anybody but those that user follows https://gbatemp.net/account/privacy
The user you may be wanting to post on the profile of might have such a setting enabled, and going through the link I started out this with from time to time there are a few of those.
You need to have a post count of 10 before you can post status updates and post on peoples profiles. This post count doesn't include posts in the introductions area or the Edge of the Forum (and some other non-forum areas)