r-api-3.5 is included in r-base-core...
You added this repository:
sudo add-apt-repository ppa:c2d4u.team/c2d4u4.0+
And ran this command:
sudo apt update && sudo apt install r-cran-rstan -d
Correct?
I just tested this on my machine - and it installed. The above includes r-base-core as well.
Are you set to Main Server? Or am I missing something here?
Otherwise... Here is r-base-core (I doublecheckd... amd64)
http://archive.ubuntu.com/ubuntu/pool/universe/r/r-base/r-base-core_3.6.3-2_amd64.deb
This must be driving you up the wall in frustration.
