This part makes me think that this is the Rccp install problem.
My understanding is the different versions of R have different clang versions and fortran versions.
You can look in your usr files to see if you have them (this repo, macOS r tools has a package that will try to detect and download the correct files directly from the places they are held.
This is all complicated by things I don’t fully understand, for instance, The most current version of gfortran for Mojave is 8.2. However this is not the version R needs which is 6.1
R 3.6 needs clang7.
The issues page on the repo above has details about the versions. If you look in your /usr/local/ you should see if you have the right version of clang installed. I think the pkg I linked will install the correct version of fortran.
I did some sort of hack job putting it all together and fixing my problems but I did eventually succeed (I had this problem unrelated to Stan, it was a problem that also affected installing any r packages that weren’t already compiled.).
There is another long thread here that addresses various Mac problems.
Unless someone comes here and tells you I am an imbecile and this isn’t related to the stuff I’ve been talking about–
FIRST I would check out that last post and see if I have all the files and versions installed (command line tools, Xcode, clang, fortran). IF they are all there and correct the makevars file might need to be fixed, and if not then I am wrong about everything.
I hope that helps at ALL!