The Pokemon Company has put up a very obvious notice on the official Pokemon Sword and Shield Expansion Pass website.
The company reminded users to be very careful with the Expansion Pass that they purchase – for example, if they own Pokemon Sword, they should buy the Pokemon Sword Expansion Pass, and if they only have Pokemon Shield, they should get the Pokemon Shield Expansion Pass.
If they get the wrong Expansion Pass, like say the Pokemon Shield Expansion Pass when they only have Pokemon Sword, it will not work at all, so please be careful folks.
See the infographic below if you are still unclear:
The best way to avoid this problem is by purchasing the Expansion Pass directly from Pokemon Sword/Shield.
What do you think?