The ad not appearing comment comes up so frequently and I experience it myself. I have now gone multiple days using Brave on my Mac laptop for probably 4-5 hours. Last I saw an ad was the 14th (now the 18th).
I found the following which seems to say the Brave ad policy is misleading. To this point, I agree:
The difference in wording may seem small, but there is a substantial difference conceptually. The goal is to show you privacy respecting ads that are relevant to the user in the right context (a “match” is found) – not to make sure that all users “see some ads” . Enabling Ads does not guarantee that you’re going to see ads – if the system does not find a match for you in the ads catalog, then you will not see any (more information on this below).
The policy on serving ads should be more clearly stated and should not require a user to explore the community site when nothing seems to change. By now, this issue should appear in the trouble shooting guidelines Brave makes available directly from the browser.