i need a way to have my plugin fill

[NiLSPACE]

Minecraft coordinates use y for height, so you're trying to set a block at y=500 which can't be done because Cuberite only supports heights from 0 to 255.
You can also use the E_BLOCK_XYZ enums so you don't have to declare every block.

-- Sets the block where the player is to bedrock
function PlayerJoined(Player)
    local World = Player:GetWorld()
    local position = Player:GetPosition():Floor():addedY(-1);
    World:FastSetBlock(position, E_BLOCK_BEDROCK, 0)
end