Why isn't my brick disappearing?

The problem is that you are doing

Workspace.Parent

When it really should be

script.Parent

Workspace.Parent references to the actual place as a whole. script.Parent references to the script's parent which if the script is a child of the object you are trying to manipulate, script.Parent would be the correct path.

Also, you might want to look into for loops as it makes it far easier to do transparency. You could do

script.Parent.Touched:connect(function(t)
    for i  = 0, 1, 0.1 do
        wait(.2)
        script.Parent.Transparency = i
    end
    script.Parent.CanCollide = false

    for i  = 1,0, -0.1 do
        wait(.2)
        script.Parent.Transparency = i
    end
    script.Parent.CanCollide = true
end)

Your problem is that you are attempting to access Workspace's parent, therefore you are trying to access the Game.

Instead of that, you would need to access the script's parent.

Also, there is a much more efficient way of changing the transparency, such as using a for loop or repeat loop.

Here is the corrected code, explained in comments:

print("Spleef brick")
brick = script.Parent
increment = 0.1
script.Parent.Touched:connect(function(t)

    --this repeat loop repeats all code in between the "repeat" and the "until" until the condition stated after the "until" statement. Therefore, this will repeat adding the Brick's transparency by 0.1 until the Transparency is equal to 1, or completely transparent.

    repeat
        brick.Transparency = Brick.Transparency + increment
        wait(.2)
    until
        brick.Transparency == 1

        brick.CanCollide = false
        wait(2)
        brick.CanCollide = true

    --this is the same as the above loop, except it makes it less transparent rather than more.
    repeat
        brick.Transparency = Brick.Transparency - increment
        wait(.2)
    until
        brick.Transparency == 0


end)

Now, there are other ways to do this, for example, a for loop. I just wanted to use repeat in this instance, as it is easier for beginners to understand.

Hope I helped!

Workspace.parent is your problem if you didn't know already by previous comments.