Why doesn't my code produce the Subtotal, VAT and Total values?

Question

I have been following a guide step by step to create a Tkinter management system, however, my code still doesn't output a value when I add sweet quantities and click the total button to create a subtotal, vat, and total float output.I am new to coding and can't find a solution online or in the tutorial. Thanks, Mo.

    from tkinter import*
    import random
    import time 
    
    def Ref():
        x = random.randint(1234, 434354)
        randomRef = str(x)
        rand.set(randomRef)
    
        CoG = float(galaxy.get())
        CoK = float(kinder.get())
        CoB = float(bounty.get())
        CoT = float(twix.get())
    
        Cost_Galaxy = CoG * 1.99
        Cost_Kinder = CoK * 1.49
        Cost_Bounty = CoB * 1.29
        Cost_Twix = CoT * 0.99
    
        cost_subtotal = "£", str('%.2f' % (Cost_Bounty + Cost_Galaxy + Cost_Kinder + Cost_Twix))
        cost_tax = "£", str((Cost_Bounty + Cost_Galaxy + Cost_Kinder + Cost_Twix) * 0.2)
        cost_total = "£", (Cost_Bounty + Cost_Galaxy + Cost_Kinder + Cost_Twix + cost_tax)
    
        subtotal.set(cost_subtotal)
        vat.set(cost_tax)
        total.set(cost_total)
    
    def Quit():
        root.destroy()
    
    def Reset():
        rand.set("")
        galaxy.set("")
        kinder.set("")
        bounty.set("")
        twix.set("")
        total.set("")
        subtotal.set("")
        vat.set("")
    
    root = Tk()
    root.geometry("1600x800+0+0")
    root.title("Sea Front Sweet & Candy Shop System")
    
    Tops = Frame(root, width = 1600, height = 50, bg = "light blue", relief=SUNKEN)
    Tops.pack(side=TOP)
    
    Frame_1 = Frame(root, width = 800, height = 700, relief=SUNKEN)
    Frame_1.pack(side=LEFT)
    #---------------------------Time---------------------------
    
    localtime = time.asctime(time.localtime(time.time()))
    
    #---------------------------Info---------------------------
    
    lblInfo = Label(Tops, font = ('arial', 50, 'bold'), text = 'Sea Front Sweet & Candy Shop System', fg = 'navy', bd = 10, anchor = 'w')
    lblInfo.grid(row = 0, column = 0)
    lblInfo = Label(Tops, font = ('arial', 20, 'bold'), text = localtime, fg = 'navy', bd = 10, anchor = 'w')
    lblInfo.grid(row = 1, column = 0)
    
    #---------------------------Sweets---------------------------
    
    rand = StringVar()
    galaxy = StringVar()
    kinder = StringVar()
    bounty = StringVar()
    twix = StringVar()
    total = StringVar()
    subtotal = StringVar()
    vat = StringVar()
    
    lblReference = Label(Frame_1, font = ('arial', 16, 'bold'), text = 'Reference', bd = 10, anchor = 'w')
    lblReference.grid(row = 0, column = 0)
    txtReference = Entry(Frame_1, font = ('arial', 16, 'bold'), textvariable = rand, bd = 10, bg = 'light blue', justify = 'right')
    txtReference.grid(row = 0, column = 1)
    
    lblGalaxy = Label(Frame_1, font = ('arial', 16, 'bold'), text = 'Galaxy', bd = 10, anchor = 'w')
    lblGalaxy.grid(row = 1, column = 0)
    txtGalaxy = Entry(Frame_1, font = ('arial', 16, 'bold'), textvariable = galaxy, bd = 10, bg = 'light blue', justify = 'right')
    txtGalaxy.grid(row = 1, column = 1)
    
    lblKinder = Label(Frame_1, font = ('arial', 16, 'bold'), text = 'Kinder', bd = 10, anchor = 'w')
    lblKinder.grid(row = 2, column = 0)
    txtKinder = Entry(Frame_1, font = ('arial', 16, 'bold'), textvariable = kinder, bd = 10, bg = 'light blue', justify = 'right')
    txtKinder.grid(row = 2, column = 1)
    
    lblBounty = Label(Frame_1, font = ('arial', 16, 'bold'), text = 'Bounty', bd = 10, anchor = 'w')
    lblBounty.grid(row = 4, column = 0)
    txtBounty = Entry(Frame_1, font = ('arial', 16, 'bold'), textvariable = bounty, bd = 10, bg = 'light blue', justify = 'right')
    txtBounty.grid(row = 4, column = 1)
    
    lblTwix = Label(Frame_1, font = ('arial', 16, 'bold'), text = 'Twix', bd = 10, anchor = 'w')
    lblTwix.grid(row = 5, column = 0)
    txtTwix = Entry(Frame_1, font = ('arial', 16, 'bold'), textvariable = twix, bd = 10, bg = 'light blue', justify = 'right')
    txtTwix.grid(row = 5, column = 1)
    
    #--------------------Pricing------------------
    
    lblSubtotal = Label(Frame_1, font = ('arial', 16, 'bold'), text = 'Subtotal', bd = 10, anchor = 'w')
    lblSubtotal.grid(row = 1, column = 2)
    txtSubtotal = Entry(Frame_1, font = ('arial', 16, 'bold'), textvariable = subtotal, bd = 10, bg = '#ffffff', justify = 'right')
    txtSubtotal.grid(row = 1, column = 3)
    
    lblVat = Label(Frame_1, font = ('arial', 16, 'bold'), text = 'VAT', bd = 10, anchor = 'w')
    lblVat.grid(row = 2, column = 2)
    txtVat = Entry(Frame_1, font = ('arial', 16, 'bold'), textvariable = vat, bd = 10, bg = '#ffffff', justify = 'right')
    txtVat.grid(row = 2, column = 3)
    
    lblTotal = Label(Frame_1, font = ('arial', 16, 'bold'), text = 'Total', bd = 10, anchor = 'w')
    lblTotal.grid(row = 4, column = 2)
    txtTotal = Entry(Frame_1, font = ('arial', 16, 'bold'), textvariable = total, bd = 10, bg = '#ffffff', justify = 'right')
    txtTotal.grid(row = 4, column = 3)
    
    #-------------------Buttons------------------
    
    
    btnTotal = Button(Frame_1, padx = 16, pady = 8, bd = 10, fg = 'black', font = ('arial', 16, 'bold'), text = 'Total', 
    bg = 'light blue', command = Ref).grid(row = 7, column = 1)
    
    btnReset = Button(Frame_1, padx = 16, pady = 8, bd = 10, fg = 'black', font = ('arial', 16, 'bold'), text = 'Reset', 
    bg = 'light blue', command = Reset).grid(row = 7, column = 2)
    
    btnQuit = Button(Frame_1, padx = 16, pady = 8, bd = 10, fg = 'black', font = ('arial', 16, 'bold'), text = 'Quit', 
    bg = 'light blue', command = Quit).grid(row = 7, column = 3)
    
    
    root.mainloop()

Answer 1

the problem is that in cost_total you are using cost_tax to calculate the total value, but cost_tax is a string and not a float. Your program will work if you change this line by:

cost_total = "£", str((Cost_Bounty + Cost_Galaxy + Cost_Kinder + Cost_Twix)*1.2)

which is the same thing of summing the subtotal with the tax that is 20% of the subtotal