Hi, Davide-
It's pretty hard to diagnose and remedy without more basic information about your system. Is it Mac or Linux (or Windows -> Linux)? What version of R do you have? If you could provide the output of
afni_system_check.py -check_all
that would probably provide most of the useful information.
My *guess* is that you might have to update your version of R to 3.5.[something] to be able to get brms; we have seen that a lot lately with some Bootcamp preparation. As to the sf, I'm not sure, but perhaps it is a similar issue. Again, this is just a guess without more info.
--pt
