There are some errors with your code. First of all, you should either make the button first and then grid it, or you can directly grid it. In your code, you are making a variable called password_button, but at the end of it, you are trying to grid the button from inside the variable. To solve the problem, you can just move the grid command to a separate line like this:
password_button = Button(window,
text="SUBMIT",
width=5,
bg="black")
password_button.grid(row=2, column=0,sticky=W)
Second of all, I don't think that changing background colors of buttons in tk is possible, but you can use ttk widgets to change the background color of a button.