When I try to use the Morty Card, it gives me this error: Oh No! You've already got this buff!
What does it mean?

It means that you have either already used a card for this month, you collected your interest for the day, or you are already at the max interest. There is a current cap at 3%. There is an open topic about the interest and vaults. Feel free to post there!
Dexter by ❤