I just had a similar headache because of some system updates on Ubuntu 20.04. After a long meandering path, I eventually managed to sort out all the hiccups.
Try the following command inside R:
install.packages("Rcpp", dependencies = TRUE, INSTALL_opts = '--no-lock')
install.packages("brms", dependencies = TRUE, INSTALL_opts = '--no-lock')
Paste here the error message in case that occurs.
Gang
Edited 3 time(s). Last edit at 09/23/2021 09:08AM by ptaylor.