Unfortunately checking for this problem it doesn't show and I can successfully add an item to the cart so it looks like either you fixed this and didn't update the post or changed the inventory of the item so others cannot test it.
If you need further help you need to make a test product so that others can inspect the issue.
If you,staff or collaborators recently modified your theme or installed an app that changed your theme try a rollback:
Thanks for the reply, but I just tried and it still does not work. When I select just any size, it will all turn to XL, but when i select the unavailable size- Xs, S, L ( inventory 0 ) with color, it will show the error 404. Thank you!.