The price of NVMe disk getting closer or similar to SSD, most recent servers have only NVMe disks.

Unfortunately, installing Ubuntu-18.04 on such server triggers the error

Validation error: 'nvme.XXXXXX' is not valid under any of the given schemas in [...]

This issue has been fixed in https://bugs.launchpad.net/ubuntu/+source/curtin/+bug/1840524/comments/8 but is not yet backported on 18.04.3

If, you need to install 18.04, because for example of the NVIDIA driver support, the fix is to:

1. flash a usb dongle with ubuntu-18.04
2. after booting on the usb dongle, go to a terminal with Alt+F2 and enter

   sudo snap refresh --edge subiquity

3. next, go back to main screen with Alt+F1 and start the installation.

Hopefully, 18.04.4 should have the patch.