At this point, you have the option of using that Prompt to your advantage.
Completely remove Nvidia drivers:
sudo apt remove --purge '^nvidia-.*'
Ensure you have the repository for the Nvidia 555 driver:
sudo add-apt-repository ppa:graphics-drivers/ppa
Install the 555 driver (using DKMS for Dynamic Kernel Module Support):
sudo apt update && sudo apt install nvidia-driver-555 nvidia-dkms-555
I recommend using the X-Swat team updates... Mesa and the like...
sudo add-apt-repository ppa:ubuntu-x-swat/updates
sudo apt update && sudo apt full-upgrade