A few people over the last year have asked me why they can’t post in some categories. First off, it’s nothing anyone is doing wrong and, other than what comes with a person’s trust level, there’s no restriction on any individual members. The site is working correctly as it’s set up.
To try and keep things organised, some sub-categories are grouped under a read-only category, like the Marketplace. Others are grouped but you can post in the main category, like The Workshop. This inconsistency can lead to confusion and frustration. It’s not just you, @Joe. 
I’ll do some work on the weekend to try and make it clearer when you’re looking at a particular category why you can’t post in it and what to do. In the slightly longer term I plan to look across the whole site, now that it’s been running for a while, to see where sections can be consolidated and how to make the structure more obvious and easier to navigate, especially for new people
Finally, it’s worth having a look at the Guides & FAQs category occasionally for anything that answers a question you may have after using the forum for a while. If you think something should be added to that help library, let me know.