I want to install pelican to create my online website. However, to install it on OVH server where my website is hosted, I need to run command lines but i can only host files and folders in /www/ on my server. How can I install pelican in this case ?
You can certainly install Pelican on a server if you want to do so. But there's no need to install Pelican on the server; it's meant to be installed on your local computer. Once installed locally, you generate your site and then transmit the output to your server. Many folks (myself included) use rsync to upload the generated HTML/CSS/JS to the server from which the site is actually served (OVH, in your case).
Once Pelican has been installed locally, you can run pelican-quickstart to create a skeleton project, answering the relevant questions for SSH/rsync. If you install Invoke as well, you can then edit the provided tasks.py and run invoke publish to automatically generate your site and transmit it to /var/www/yoursite/.
If for some reason you want to install Pelican on your server and generate the output there, the installation instructions would be much the same as installing Pelican on your local workstation.