wget -O - https://github.com/OpenMediaVault-Plugin-Developers/packages/raw/master/install | bash

If you get a 404 when trying to access it via system, do:

Please try ctrl-shift-R 

https://wiki.omv-extras.org/doku.php?id=omv6:omv6_plugins:kvm

One needs to enable backports in omv-extras

sudo apt install tigervnc-viewer