[Python] ImportError: No module named 'Tkinter' [duplicate]

For some reason, I can't use the Tkinter (or tkinter, on Python 3) module. After running the following command in the python shell:

import Tkinter


or this, in Python 3:

import tkinter


I got this error

ModuleNotFoundError: No module named 'Tkinter'
​

or this:

ModuleNotFoundError: No module named 'tkinter'
​

What could be the reason for these errors and how can I solve it?

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