Thank you for your answer.
Yes I've already seen this page, and I installed the package 'rmarkdown' as they said on it.
Yes it was my first time on Rmarkdown, I just created a new page 'File' -> 'New file' -> 'R Markdown'., and the default code appeared on this new page.
I wanted to run it but when I tried, this error appeared, and I don't understand why.
Is there an ohter package I should install?