I believe that these cards are only useable when you are playing in Garden Ops. It should have a clickable icon that says Self Revive after you are downed.
As per IGN : "f the Zombies knock out one of the players on the team, teammates have 20 seconds to get to and revive them. The downed player can also use a Self Revive consumable they may have received from a pack in the Sticker Shop. Only three Self Revives can be used per game. All knocked out players will be automatically revived at the start of the next wave."
"If all players are downed, it's game over unless one player uses a Team Retry consumable they may have received from a pack in the Sticker Shop. Only three Team Retries can be used per game. That's three total, not three per player."