There is a conflict between the driver CUDA wants to use (nvidia-390)
and the one you have installed.
I had the same problem: "which is also in package nvidia-340..."
For me this was solved by first purging the old nvidia driver
> sudo dpkg -P nvidia-340
Only then would
sudo apt --fix-broken install
work. You can purge everything via
sudo apt purge nvidia*
sudo apt purge cuda*
You can move on to get the newer nvidia drivers
sudo apt install nvidia-390 nvidia-390-dev
Installing the nvidia-384 drivers should also work, but I'm not sure about that.