I’m running Catalina (unfortunately). running xcode-select --install seems to have fixed the xcrun error (at least it did for installing cmdstanr which failed with a similar error initially). Unfortunately, I’m still waiting for that to finish compiling and haven’t been able to try install.packages(“rstan”, type = “source”) yet. As soon as cmdstanr is finished, I’ll give that a shot and post an update.
Thanks