Rmarkdown book compilation bug

Dear fellow Rmarkdown users,

I have downloaded the latest Rmarkdown book from github:

I am able to compile it as gitbook but not as pdf and pdf2.

For pdf, the error is:

! Package fontspec Error: The font "Palatino" cannot be found.

I have installed texlive 2018 and by querying it, palatino font is installed. I checked using:

tlmgr info palatino

Anyone with tex knowhow have any idea what is the problem?

Compiling as pdf2, the error is:
Error in get(name, envir = asNamespace(pkg), inherits = FALSE) :
object 'strip_latex_body' not found
Calls: ... render_cur_session -> -> -> post -> ::: -> get
Please delete _main.Rmd after you finish debugging the error.

Anyone with some idea what is wrong there?

Thanks in advance,