Its not really documented in the in the “New Members Start here” thread, but, Discourse uses some basic warm up / trust settings to prevent bots / spam etc… (@JustinAtSense, maybe worth an update on that page as this trust stuff comes up from time to time)
You can read about them at the below link. I’m not entirely sure how Sense has tweaked them as they can be customized, but the defaults seem to line up with some of what I’ve seen people here discover.
But TL;DR - You have to take certain actions on the forums as a new user to earn trust to gain further access. So Read X posts, or reply to a forum before getting the ability to start new threads. Or post a certain number of times before you can post images etc…