S0E21: Rick and Morty in the Eternal Nightmare Machine

I don't have specific information on an episode labeled "Rick and Morty - S0E21: Episode 21," as there doesn't appear to be a special or episode with that title in the "Rick and Morty" series up to October 2023. If you meant a different episode or if it's a newer release, I suggest checking the latest available episodes or specials for the details. If you have more context or a specific episode title in mind, feel free to share!