You have a Catalina problem. If you have not done so already, follow
and then to install the C++ toolchain and then in a fresh R session do
install.packages("rstanarm", type = "source")
You have a Catalina problem. If you have not done so already, follow
and then to install the C++ toolchain and then in a fresh R session do
install.packages("rstanarm", type = "source")