Hi Kris,
I struggled with the same issue on my computer for days and it turned out to be an issue with using the newest version of R along with rstan and brms. I had to uninstall R, reinstall version R 4.1.3, then run the code below for installation. After I did that, it worked just fine.
#install.packages("StanHeaders", repos = c("https://mc-stan.org/r-packages/", getOption("repos")))
#install.packages("rstan", repos = c("https://mc-stan.org/r-packages/", getOption("repos")))
#install.packages("brms")
#install.packages("httpuv")
#install.packages("StanHeaders")
#install.packages("shinystan")
#install.packages("igraph", type="binary")