[Tkinter] .delete and .insert not working

[Yoriz]

You need to get the reference to the object Entry, by calling .grid on it you lose it, call grid separately.
change

display = Entry(root, width=25).grid(row=0, column=0, columnspan=3, padx=10, pady=20)

to

display = Entry(root, width=25)
display.grid(row=0, column=0, columnspan=3, padx=10, pady=20)