Why is it calling ImageTransparency a nil value?

Your script doesn't work because B is a table. If you want to do this for every item in the table, you can loop through with a for loop using in pairs (see below). Another issue with your script is that you're manually going through the ImageTransparency in a loop when you could simply tween from one value to another.

(Comments of script should have a fine explanation but if you want more information on Tweens or GetChildren then check the wiki)

local parent = script.Parent -- renamed since P is unclear
local parentsChildrenList = parent.Parent:GetChildren() -- instead of defining it with each click, define it ahead of time

local linearTweenSpecs = TweenInfo.new(0.8, Enum.EasingStyle.Linear) -- take 0.8 seconds to fade out and have change of transparency consistent
local tweenService = game:GetService("TweenService") -- to help with tweening so you don't need loops like you were using
local transparency1Tween = {ImageTransparency = 1} -- so that we don't redefine it each tween

parent.MouseButton1Click:Connect(function()
    for _,image in pairs(parentsChildrenList) do
        if image.ClassName == "ImageLabel" then -- if it's an imagelabel and not a textbox or something
            local transparencyTween = tweenService:Create(image, linearTweenSpecs, transparency1Tween) -- create a tween using the variables we defined earlier
            transparencyTween:Play() -- play the tween
            spawn(function() -- in a new thread (so all images can tween at same time)
                transparencyTween.Completed:Wait() -- wait for the tween to complete
                image.Visible = false -- make Visible false (like it was earlier)
            end)
        end
    end
end)

The other answer's solution will not work. The indexes in the table returned by a :GetChildren() call are numbers that aren't related to the instance since it is a table, not a dictionary.

Edit: Added wiki links

Edit 2: I was wrong about the "meaningless numbers" part, updated answer to be more clear on what I originally meant. (Not related to the instance rather than completely random and meaningless.)

the reason is that B is a table, not an image label/button. To change that be can do this :

P = script.Parent

P.MouseEnter:connect(function()
    P.Size = P.Size + UDim2.new(0.06, 0, 0.03, 0)
    P.Position = P.Position - UDim2.new(0.03, 0, 0.015, 0)
end)

P.MouseLeave:connect(function()
    P.Size = P.Size - UDim2.new(0.06, 0, 0.03, 0)
    P.Position = P.Position + UDim2.new(0.03, 0, 0.015, 0)
end)

local function cycle (table,thing)
    for i,v in pairs(table) do
        if v == thing then return true end
    end
    return false
end

P.MouseButton1Click:connect(function()
    local image P.Parent:FindFirstChild("ImageLabel") or P.Parent:FindFirstChild("ImageButton")--maybe use find first descendant 
    if image then
        for i=1, 20 do
            wait(0.04)
            image.ImageTransparency = image.ImageTransparency + 0.05
        end
        image.Visible = false
    end
end)

so this checks the table for "ImageLabel" and sets its visibility and transparency