To activate the rich text editor you have to log out and then in again after selecting it for the changed setting to be recognized, since it's not really built into the site but a plugin from elsewhere.
I'd recommend just using HTML though if you intend to have any scripting in your game, the RTE can cause some issues with that due to some of the special characters it inserts. And personally I just find it to be a cluttered looking ugly box that loads slowly and doesn't play well with dark mode.
