Ah, that's the problem then! Herobrine does not yet support Spigot 1.17. If you check towards the beginning of the server log, you should see a message that says: "This version of Herobrine is not compatible with this server's Spigot version and will be disabled."
Additionally, if you try running the "/plugins" command, Herobrine will be marked as disabled. My recommendation is to keep checking back for the release of a new version of the plugin that supports Spigot 1.17. I have not had a chance to get around to updating it yet, but I hope to very soon!