Tkinter module not found on Ubuntu
What worked for me in Ubuntu was actually just:
sudo apt-get install python3-tk
For python 3.6:
sudo apt-get install python3.6-tk
I didn't read anywhere, I simply tried it, as onteria_'s method didn't seem to work for me.
The answer to your question is that Tkinter is renamed to tkinter in python3
that is with lowercase t