How to remove Python 3.6 completely from Ubuntu 18.04

Don't do that , just don't.
you'll have regrets a lot of it.
And I'm serious.
A lot of things will deleted,but by the way, if you already done that don't worry, there is a solutions.
Just run this code in your terminal

sudo apt install ubuntu-desktop

And if you want you can run this one too

#ubuntu-18.04.1
sudo apt install $( curl http://releases.ubuntu.com/18.04/ubuntu-18.04.1-desktop-amd64.manifest | sed -e 's#\t.*##g' )

Link for reference: https://askubuntu.com/questions/1087267/how-do-i-reinstall-the-software-packages-that-came-with-ubuntu-18-04

Ubuntu 18.04 requires python 3.6 to function. If you remove python 3.6, you won't be able to log in after a restart. Ubuntu will still have python 3.6minimal, even after uninstalling python 3.6. so you will have to remove both. Once that is done, you'll get an error that looks like this.

https://itsfoss.com/failed-to-start-session-ubuntu-14-04/

You can follow the above steps if you break Ubuntu.