Q:

I am not sure why it is not showing the message 'Start Calculating'

I am writing a calculator application and using Tkinter as well as python 3.7. I am currently stuck as when I try and run it, everything works except that it does not show 'Start calculating...' in the screen. Is anyone able to help please?
I have tried calling the function used at the end but that did not work. I am not sure what else to try.
from tkinter import *

def btn(numbers):
    global operator
    operator = operator + str(numbers)

root = Tk()
root.title("Calculator")

operator = ''
txt_input = StringVar(value='Start Calculating...')

#========================Screen=======================
Display = Entry(root, fg='white', bg='white', 
    justify='right', bd=28, textvariable=txt_input)
Display.grid(columnspan=5, sticky='NSEW')

#=======================Row1==========================
b7 = Button(root, padx=30, pady=15, bd=8, fg='black', text='7', highlightbackground='white', highlightthickness=0.0001).grid(row=1, column=0, sticky='NSEW')
b8 = Button(root, padx=30, pady=15, bd=8, fg='black', text='8', highlightbackground='white', highlightthickness=0.0001).grid(row=1, column=1, sticky='NSEW')
b9 = Button(root, padx=30, pady=15, bd=8, fg='black', text='9', highlightbackground='white', highlightthickness=0.0001).grid(row=1, column=2, sticky='NSEW')
clear = Button(root, padx=30, pady=15, bd=8, fg='black', text='C', highlightbackground='green', highlightthickness=0.0001).grid(row=1, column=3, sticky='NSEW')

#=======================Row2==========================
b4 = Button(root, padx=30, pady=15, bd=8, fg='black', text='4', highlightbackground='white', highlightthickness=0.0001).grid(row=2, column=0, sticky='NSEW')
b5 = Button(root, padx=30, pady=15, bd=8, fg='black', text='5', highlightbackground='white', highlightthickness=0.0001).grid(row=2, column=1, sticky='NSEW')
b6 = Button(root, padx=30, pady=15, bd=8, fg='black', text='6', highlightbackground='white', highlightthickness=0.0001).grid(row=2, column=2, sticky='NSEW')
plus = Button(root, padx=30, pady=15, bd=8, fg='black', text='+', highlightbackground='orange', highlightthickness=0.0001).grid(row=2, column=3, sticky='NSEW')

#=======================Row3==========================
b1 = Button(root, padx=30, pady=15, bd=8, fg='black', text='1', highlightbackground='white', highlightthickness=0.0001).grid(row=3, column=0, sticky='NSEW')
b2 = Button(root, padx=30, pady=15, bd=8, fg='black', text='2', highlightbackground='white', highlightthickness=0.0001).grid(row=3, column=1, sticky='NSEW')
b3 = Button(root, padx=30, pady=15, bd=8, fg='black', text='3', highlightbackground='white', highlightthickness=0.0001).grid(row=3, column=2, sticky='NSEW')
minus = Button(root, padx=30, pady=15, bd=8, fg='black', text='-', highlightbackground='orange', highlightthickness=0.0001).grid(row=3, column=3, sticky='NSEW')

#=======================Row4==========================
b0 = Button(root, padx=30, pady=15, bd=8, fg='black', text='0', highlightbackground='white', highlightthickness=0.0001).grid(row=4, column=0, sticky='NSEW')
dot = Button(root, padx=30, pady=15, bd=8, fg='black', text='.', highlightbackground='orange', highlightthickness=0.0001).grid(row=4, column=1, sticky='NSEW')
div = Button(root, padx=30, pady=15, bd=8, fg='black', text='/', highlightbackground='orange', highlightthickness=0.0001).grid(row=4, column=2, sticky='NSEW')
times = Button(root, padx=30, pady=15, bd=8, fg='black', text='x', highlightbackground='orange', highlightthickness=0.0001).grid(row=4, column=3, sticky='NSEW')

#=======================Row5==========================
equals = Button(root, padx=95, pady=15, bd=8, fg='black', text='=', highlightbackground='green', highlightthickness=0.0001).grid(row=5, columnspan=2)
bracket1 = Button(root, padx=35, pady=15, bd=8, fg='black', text='(', highlightbackground='orange', highlightthickness=0.0001).grid(row=5, column=2)
bracket2 = Button(root, padx=38, pady=15, bd=8, fg='black', text=')', highlightbackground='orange', highlightthickness=0.0001).grid(row=5, column=3)

root.mainloop()

A:

Your problem is the foreground (text) and background colour are both white so you wouldn't see any text in it regardless.
As a side note:
To place a value in an entry, you can also use the insert entry method if you don't want to associate it with a variable:
Display.insert(0, 'Start Calculating...') or Display.insert(0, txt_input.get())
The 0 is the index (location) of where in the entry's text you want to insert the specified text i.e. insert the text at the 0th position - the start.

