@Chiraz_BenAbdelkader Can you try installing from these updated links
install.packages(
"https://artifacts.r-hub.io/StanHeaders_2.21.0-6.tar.gz-5af2069f840e9b971ee5b0e175da5ea6/StanHeaders_2.21.0-6.zip",
repos = NULL, type = "win.binary")
install.packages(
"https://artifacts.r-hub.io/rstan_2.21.2.tar.gz-3145e4a7fcd4016d378efd0e537d8b91/rstan_2.21.2.zip",
repos = NULL, type = "win.binary")
and let us know if you still get errors? Thanks!
Edit: these are the same links as the updated links in the post @bgoodri linked to. I hadn’t seen his response.