It does seem to come up an awful lot, but I’m not sure editing the FAQ/rules is a solution (FAQ - Anki Forums). Doesn’t that seem like it’s a long way from where the issue is happening? There are cues right on the page that folks are missing.
We address this in the About/pinned post in the Anki parent category, but not in the AnkiMobile and AnkiDroid parent categories, so that could be updated.
I don’t know if that “you’re not allowed …” message is customizable, but I agree that an update to that could help too.
[I edited your title because I didn’t understand the relevance of the “15 characters” part.]
You are right about the cues, but often, people don’t look into it as much as they should and then subsequently miss them.
Also, what all count as top-level categories?
I came here through Google to realize how to write a post. Apparently I posted once before, but after coming back again, I had no idea why it told me I couldn’t. I thought I was banned for something, and was about to open a new account. People don’t play detectives about “categories” and “subcategories” when they’re trying to post a message. That error message is misleading at best, because “can’t post in a category” also implies that you can’t post in any of its subcategories. If you can’t enter a country, it makes no sense to then try entering any of its cities. It should say “can’t post in top-level category”. Also, like every error message, it should give a reasonable suggestion of what to do instead, like saying “Pick a subcategory”.
I’m not sure much can be done on our end with the current setup, except tweaking our topic descriptions to make this more obvious. We could move the help topic to the top level, but then it’s no longer possible to display help questions separately from the other subcategories.