Can professor say how they got it installed?
Are you set to Main Server in Software & Updates?
More to try:
sudo apt -y install r-base gdebi-core
Grab this .deb file:
https://download1.rstudio.org/desktop/bionic/amd64/rstudio-2021.09.0-351-amd64.deb
You can open it with gdebi or
sudo gdebi rstudio-2021.09.0-351-amd64.deb
If it works... try launching...
rstudio