why won't my function connect and text appear?

The text doesn't appear because you are never updating its transparency. In the code

function appear(transparency)
    print("appear function called")
    for i = 1, 0, .05 do
    transparency = i
    end
end 

you are only updating the variable transparency and not the actual transparency of the label's text. To make this work, consider using this function

function appear(label)
    print("appear function called")
    for i = 1, 0, .05 do
        label.TextTransparency = i
        label.TextStrokeTransparency = i
        wait()
    end
end 

and calling it by passing the label mc as an argument. Do similarly for disappear. Keep in mind you will want to call wait upon each loop iteration to prevent the text from appearing instantly.

You get the Value of the Transparency and not set it to the new value and you did this loop:

for i = 1, 0, .05 do

this will not decrease i so it gets visible again you need to do the loop like this:

for i = 1, 0, -.05 do

try this :

function appear(object)
    print("appearing")
    for i = 1, 0, -.05 do
        object.TextTransparency = i
        object.Transparency = i
    end
end

function dissappear(object)
    print("dissappearing")
    for i = 0, 1, 0.5 do
        object.TextTransparency = i
        object.Transparency = i
    end
end

function ChangeOfMoneyGui(clicker)
    db = 0
    print("connected")
    if db == 0 then
        db = 1
        print("debounce")
        local plrgs = clicker:FindFirstChild("PlayerGui")
        local mc  = plrgs.ChangeOfMoney.MoneyChangeFrame.MoneyChange
        visible = false
        if visible == false then
            print("visible now")
            visible = true
            appear(mc)
            wait(2)
            dissappear(mc)
            visible = false

        end
    end
end

game.Workspace.ClickyClick.ClickDetector.MouseClick:connect(ChangeOfMoneyGui)