The most common error seems to be (or similar).
E> ld: warning: directory not found for option '-L/usr/local/gfortran/lib'
E> ld: library not found for -lgfortran
E> clang: error: linker command failed with exit code 1 (use -v to see invocation)This indicates that the fortran compiler is missing. This can can be installed here (the .dmg file).
Typically the above has solved the issue. If not, then an additional error could be
Error: (converted from warning) Setting LC_CTYPE failed, using "C"This was solved by updating both R and R-studio. More information can be found here.
If that does not work, then perhaps Xcode is missing. This can be installed at the “Mac App Store”.