It appears that either your mirror.switch.ch server is missing the required files, or you don't have the required channel enabled.
To fix both of those, please:
1. Open app "Software & Updates"
2. Tick the first 4 out of 5 checkboxes.
3. Change "Download from" to "Main server".
4. Close the app "Software & Updates".
5. Run: sudo apt update
6. Run: sudo apt install nvidia-driver-390
If that fails then please paste the full output of the failed command including the command line you typed.
It appears that either your mirror.switch.ch server is missing the required files, or you don't have the required channel enabled.
To fix both of those, please:
1. Open app "Software & Updates"
2. Tick the first 4 out of 5 checkboxes.
3. Change "Download from" to "Main server".
4. Close the app "Software & Updates".
5. Run: sudo apt update
6. Run: sudo apt install nvidia-driver-390
If that fails then please paste the full output of the failed command including the command line you typed.