"\\\\ds.fordham.edu/DFS/Users/mhimmelstein/My Documents/R/win-library/4.0/StanHeaders/include/stan/math/prim/mat/fun/Eigen.hpp"
For whatever its worth, this is the workstation in my office and I have admin access to the C:\ drive as well, though installing to the C:\ library path
"C:/Program Files/R/R-4.0.2/library"
Causes the same error with the local path (taken from your suggestion in this thread: Compilation error stan/math/prim/mat/fun/Eigen.hpp: No such file or directory - #10 by Aidan1)