No Build Book button in RMarkdown

Dear RStudio-Community,

for my master thesis I wanna use RMarkdown with the bookdown package. I went through the bookdown-demo template and everything was fine. Then I wanted to create it from scratch and I am not able to create the BUILD BOOK option. It is always the BUILD Website option.
I have created a new project and a new file (index.rmd) with the following code:

title: "Untitled"
author: "your name"
date: "6/3/2021"
output: downbook::gitbook
site: downbook::downbook_site

I saved it and restarted the project. It shows me the Build pane but it is always Build Website and then i get the following error:
==> rmarkdown::render_site(encoding = 'UTF-8')

error in rmarkdown::render_site(encoding = "UTF-8") :
No site generator found.

Exited with status 1.

Does anyone know the problem? Thanks in advance.

Pretty sure “downbook” needs to become bookdown three times in that little section.

Thank you for your answer.
Could you please specify it?

Do you have this in your Rmd document ?

I think what @jtbayly meant is that it should be this

title: "Untitled"
author: "your name"
date: "6/3/2021"
output: bookdown::gitbook
site: bookdown::bookdown_site

Package name is bookdown not downbook we believe.

Did you check that ?

Oh my god! You are completely right. Now it works. Thanks a lot!