Make a block that appears when another block is pressed?

Since you provided a such roughly question, I will help the code while explaining to you.

1.Define two bricks.

local BrickDisappear = workspace.PartA
local BrickTouch = workspace.PartB

The codes above defines two bricks.

2.Set up the signal(event) listener.

local Signal = nil
Signal = BrickTouch.Touched:Connect(function()
    BrickDisappear.Transparency = 1
    Signal:Disconnect()
    Signal = nil
end)

Since the Touched signal should only be listened once, we disconnect it after it fires.

3.Put all together.

local BrickDisappear = workspace.PartA
local BrickTouch = workspace.PartB
local Signal = nil
Signal = BrickTouch.Touched:Connect(function(part)
    BrickDisappear.Transparency = 1
    Signal:Disconnect()
    Signal = nil
end)

Documentation:

BasePart.Touched

BasePart.Transparency