Hi,
So I've triend to use other tutorials with no success. I thought I will post my situation for others. In my case I wanted installing OMV5 on top of debian 10 because out of the box install doesn't allow to patrition the drive and uses whole drive for system purposes (apparently you can't make shares on system drive, there is some workaround with symlinks posted but it didn't work in my case). When installing Debian I've choosen system utilities, ssh server packages and nothing else (unmark print server and visual enviroment)
If you installed Debian from DVD like me you will have to comment out "# deb cdrom: ..." to get apt-get update working
then you create file vi /etc/apt/sources.list.d/openmediavault.list with one line: deb https://packages.openmediavault.org/public usul main
next steps:
1. wget -O archive.key http://packages.openmediavault.org/public/archive.key
2. apt-key add archive.key
3. export LANG=C.UTF-8
4. export DEBIAN_FRONTEND=noninteractive
5. export APT_LISTCHANGES_FRONTEND=none
6. apt-get update
7. apt-get install postfix openmediavault
8. omv-confdbadm populate
9. ip a |grep inet (for getting your local ip address)
10. go to http://your_ip
11. Login with: admin, p: openmediavault
if you want to install omv extras do: wget -O - http://omv-extras.org/install | bash
Enjoy