It is treating the book as if it has never been published on book down before
Can you precise where you are publishing this ? bookdown.org or you own RStudio connect ?
Did you keep the rsconnect folder that was created after the first publication ? you need to transfer it with the file inside to your desktop. Otherwise, the RStudio IDE won't be able to guess taht the content has already been publish.
You can also try to use the rsconnect
and provide the url and appId of your content in your rstudio server so that the content is published in the same id.
Hope it helps