Working with the terminal using a command interface is very fast and effective way of doing stuff. But have you tried working with the terminal, listening to a music via headphones and having a terminal warning or tab-completion beep / bell turned ON at the same time? Well it can be very annoying. Listening to the PC Speaker beeps via headphones is very close to a feeling of hitting your head with a hammer. Even without headphones this beeping bell sound is very annoying.
Although you’ve got the errors you’re still able to connect to the machine:
and install the ifupdown package
apt-get install ifupdown
and reload the machine.
This guide explains how to set up software RAID1 on an already running LVM system (Ubuntu 18.04). The GRUB2 bootloader will be configured in such a way that the system will still be able to boot if one of the hard drives fails (no matter which one).
I do not issue any guarantee that this will work for you!