/etc/netplan: No such file or directory
Solution 1:
Netplan is shipped in the netplan.io package. Running apt install netplan.io will give you the /etc/netplan directory; you will need to provide your own config files in this directory, with a .yaml extension.
If the netplan.io package was not installed for you on upgrade from Ubuntu 16.04 to Ubuntu 18.04, then it appears you are missing the ubuntu-minimal package. It is recommended that you install this package to ensure you have the standard Ubuntu experience.