Updating R 3.5.3 to R 4.0.2 on Ubuntu 18.04

You have to remove problematic PPA (it does no publish packages for 18.04 LTS - bionic) first with:

sudo add-apt-repository -r ppa:grass/grass-stable

and then update your package lists

sudo apt-get update

and install new updates with their dependencies by

sudo apt-get dist-upgrade

You have added R 3.5 repository (see bionic-cran35 in its URL), instead of needed R 4.0 (bionic-cran40). You have to remove wrong repository with

sudo add-apt-repository -r 'deb https://cloud.r-project.org/bin/linux/ubuntu bionic-cran35/'

and then add correct one:

sudo add-apt-repository 'deb https://cloud.r-project.org/bin/linux/ubuntu bionic-cran40/'
sudo apt-get dist-upgrade
sudo apt-get install r-base r-base-core r-recommended