I got the error when trying to apt install cuda.
The following packages have unmet dependencies: cuda : Depends: cuda-10-1 (>= 10.1.105) but it is not going to be installed. this fixed it:
sudo apt-get clean; sudo apt-get update; sudo apt-get purge cuda; sudo apt-get purge nvidia-*; sudo apt-get autoremove; sudo apt-get install cuda;