The R spkg does not build from source on many major platforms including ubuntu-bionic-minimal, ubuntu-eoan/focal-minimal, debian-buster/bullseye/sid-minimal, linuxmint-19.3-minimal, archlinux-latest-minimal (see
https://groups.google.com/d/msg/sage-release/eMQIlhglN98/bTwDGD9HBQAJ)
This stops Sage from building on these platforms because R is currently a standard package.
I propose downgrading R to "experimental".