Nightbot syncs active chatters in your Discord server every 15 minutes. Those active users will need to have their Twitch account linked to their Discord account for Nightbot to sync the role to them. Additionally, Nightbot’s role must have permission to manage roles and must be higher than the role you’re trying to sync.
You can manually assign the user the role if you want to “force” it. Assuming the server is properly setup and the user is linked to Twitch from Discord, Nightbot won’t remove their role on next sync.