sudo apt-get install python3-pip
sudo pip3 install virtualenv
virtualenv venv
you can use any name insted of venv
virtualenv -p /usr/bin/python2.7 venv
source venv/bin/activate
source venv/bin/activate.fish
deactivate
virtualenv -p python3 myenv
python3 -m venv myenv
could you please provide more details, like
what OS are you using
what command did you run and displayed an error
do you have any screenshots you can share