Recently though, Disney has stopped you being able to add a MagicBand to your booking for pickup at check-in altogether, even though the option still appears on the site. When you log into My Disney Experience, go to the MagicBand section and click "Edit order" it sends you to this help page which says:
How can I personalise my MagicBand?
Guests can no longer personalise their MagicBands. If you booked your 2021 holiday before 24th June 2020, you can pick up your MagicBand when you check-in at your Resort. Otherwise, you can purchase a MagicBand from your Disney Resort hotel or from World of Disney including limited-edition and custom designs.