You go to a shop to buy a scratch card, then you can scratch it in the shop you bought it from. If you try to do it from your inventory, it’s worthless.

Where did you buy it from?
Go back to that location and pick “scratch card” or something.

there are 3 places.
it depends on what scratchcard you have.
lost desert scratchcards:
and then click on “scratchcards”
ice ones:
spooky:
or if you get a rare scratchcard you can sell it instead.