Xubuntu 16.04: Can't add ppa after installing python3.6 and changing symlink

Recently I installed Anaconda with python3.6. Today I found that I couldn't add ppa repository because of python problem. The first problem was No module named 'apt_pkg'

When I run ~$ which python, I got /home/username/anaconda3/bin/python

I googled and found some ways to fix it. I tried to add alias python3=/home/username/anaconda3/bin/python at the end of my ~/.bashrc

It couldn't solve my problem, then I tried to create symlink by ~$ sudo ln -s /home/username/anaconda3/bin/python /usr/bin/python3

But that just give me another error. Now when I try to run ~$ sudo add-apt-repository ppa:nilarimogard/webupd8, I got

Traceback (most recent call last):
  File "/usr/bin/add-apt-repository", line 11, in <module>
    from softwareproperties.SoftwareProperties import SoftwareProperties, shortcut_handler
ModuleNotFoundError: No module named 'softwareproperties'

I'm using xubuntu 16.04 LTS and kinda new of it. Please suggest me how to fix this problem.