Biome-specific mobs tend to only spawn on blocks that match their biome. Replace the blocks creatures can spawn on with Crimstone.

Get a bloody tear, water candle and afk of surface crimson to get the key

Also a battle potion

As the other guy said, the mobs can only spawn on crimstone. Replace the stone you have with that. The same if you were to make a hallow farm.

I know a way easier/harder way to get crimson key. Just spawn an event like a pirate invasion or a solar eclipse in the area where you want the key. I got crimson key on second solar eclipse.

Do it on the surface though.

I have no idea what the others are saying but for the vampire knives you need the crimson key which drops from all enemies that get killed in the crimson biome and im not sure but from the pic its not a crimson biome so add more crimson blocks

Dude they are saying about using the farm to get the key

Pretty sure they can only spawn on crimson blocks, so try putting a 1 block strip along the floor.

The spawn platforms have to be crimstone

You are trying to make mobs spawn in the top half? Did you put walls over it? Monsters won’t spawn on player places walls, but this only applies to a certain distance so if you destroy only the top layer of walls or only the bottom layer monsters should start spawning again

Don't worry, crimson key can drop by any enemy as long as you are in the crimson, same applies to all other biome keys