My script is not working, for some reason the command "Mainframe" is not working. Why?

Here is my code: (sry for the length!) U can help me shorten it and add more to it to finish it.

sg = Instance.new("ScreenGui",game.StarterGui);
fr = Instance.new("Frame", sg);






fr.Size     = UDim2.new(0, 200, 0, 60);

mainFrame.Position = UDim2.new(0.0, 10, 0.0, 5);
mainFrame.Size = UDim2.new(0.0, 50, 0.0, 50);  
fr.BorderSizePixel = 1

-- Location and size set Above.



fr.BackgroundColor3 = Color3.new(255/255, 0/255, 0/255)

fr.BackgroundTransparency = 0.5

fr.BorderColor3 = Color3.new(0, 1, 0) 

-- Setting the colours of the Gui Above




fr.Name = "Shop"

fr.Style = RobloxRound 

fr.Visible = true



tl = Instance.new("TextLabel", fr)
tl.Size = UDim2.new(1, 0, 1, 0)
tl.BackgroundTransparency = 1
tl.TextColor3 = Color3.new(1, 1, 1)
tl.Text = ("Shop") 
tl.Visible = true