I came back to add a comment to a topic I had posted in before, but found no option to post a reply.
This is the topic: New forum questions and tips
It doesn’t indicate it’s locked or give me any other indication I can intuit about why I can’t post in it.
Then I tried to post this comment but found hovering over the “+ New Topic” button produced a “No Entry” icon which clearly meant I couldn’t do so. As far as I can work out, the thread I previously posted in is inside the “Website [Discussion and Technical Issues]” category, but isn’t inside either of its subcategories. Is that the issue? I looked in each of the subcategories but didn’t spot the topic in either. It was only after choosing one of the subcategories (“Website Discussion and Technical Problems”) that I was allowed to start this new topic.