@bgoodri there is an issue with the current version of Rcpp v1.0.4 on CRAN.
In short, it’s now required to compile Rcpp from source at its drat repository
The following should be used until v1.0.5 or R 4.0.0 is released (on April 24th):
install.packages("Rcpp", repos="https://rcppcore.github.io/drat")
install.packages("rstan", type = "source")
Details on the Rcpp buglet: