How to remove sidebar from Home static page?

Im using the Mon Cahier theme.

Best place to ask is on the theme’s forum here:

https://www.ads-software.com/support/theme/mon-cahier#postform

The only way to do so will be by making a child theme and then customizing the template or make yourself a front-page.php template.
Looking at the link you’ve provided, the theme you are using is not mon cahier, but Manifest.