Shit happened.

I thought I can upgrade the python to 3.6 and only use 3.6. So I can remove 2.7.

So I did :

#sudo apt-get remove python2.7

And type Y for confirmation.

The next thing happen is, I lost gimp, inkscape and many other programs. Hahahaha

So, my backbox 5 is depend in python2.7 for its backend. OK, I got it.