Q: System doesn't find virtualenv even though it is installed gaute@UPG-UBUNTU:~/grr$ sudo apt install virtualenv
Reading package lists... Done
Building dependency tree       
Reading state information... Done
virtualenv is already the newest version (15.0.1+ds-3ubuntu1).
0 upgraded, 0 newly installed, 0 to remove and 125 not upgraded.
gaute@UPG-UBUNTU:~/grr$ virtualenv ~/.virtualenv/GRR
The program 'virtualenv' is currently not installed. You can install it by typing:
sudo apt install virtualenv

I don't know what happened in the system. I assume probably somebody manually deleted virtualenv directory.

A: Thanks, Waltinator, for the hint. I solved it. It just needed to create reference folder of already installed virtualenv. I did it by doing below. 
sudo /usr/bin/easy_install virtualenv

And the easy_install python file looks like below.
#!/usr/bin/python
# EASY-INSTALL-ENTRY-SCRIPT: 'setuptools==20.7.0','console_scripts','easy_install'
__requires__ = 'setuptools==20.7.0'
import sys
from pkg_resources import load_entry_point

if __name__ == '__main__':
    sys.exit(
        load_entry_point('setuptools==20.7.0', 'console_scripts', 'easy_install')()
    )

