... the guy in the comments saying stuff like "this is a bad mod" and what-not most likely had already attempted to generate a navmesh himself, and this addon was unable to overwrite it. He, and anyone else who tried to generate their own navmeshes will need to go into their Garry's Mod directories and delete the generated navmesh before this addon can work - they may need to unsubscribe from this and re-subscribe for the changes to take effect, but maybe it'll just work without needing to unsubscribe and re-subscribe.