Story mode is a separate add-on for Grand Theft Auto Online for the Xbox Series X/S. So you have to buy GTA Online for Story Mode to work.
Right now I'm seeing that Online and Story Mode are both on sale for $10 each, so it will be $20 if you want to own everything you need to play the story. There is also a bundle with both, called Grand Theft Auto V (Xbox Series X|S) which is on sale for $31.99, so the better deal right now is to buy them both separately (I have no idea why it's like this).
Here's the link for Online:
https://www.xbox.com/en-us/games/store/Grand-Theft-Auto-Onli...Here's the link for the Story Mode add-on:
https://www.xbox.com/en-US/games/store/grand-theft-auto-v-st...Here's the link for the bundle, but as I mentioned, it appears to be more expensive to buy this for whatever reason:
https://www.xbox.com/en-US/games/store/grand-theft-auto-v-xb...