Can pre-paid Cards work on Realms PC Edition?
Solution 1:
No. Prepaid cards are only for buying Minecraft, in the form of the game.
Mojang's page for pre-paid cards says that:
Minecraft for PC/Mac can be bought via pre-paid cards found in physical stores.
This means that pre-paid cards can be redeemed for buying the actual game. If you want to buy Realms, just go to the Realms page for PC/Mac and choose a plan.