It seems that I am not being able to help you any further.
But still the following suggestions:
Probably you will have seen an earlier article in the RStudio Community. That suggests checking if the Windows path contains the R executable.
Is this first time you try to create a book (with bookdown) ?
If not, what is different?
Did you start your book by creating a new 'project with bookdown' ?
If you do this now and you click on 'Build|Build_all' do you get the same error?