I tough these quicksteps could be useful for beginner 
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Download LibreNMS OVA on your local workstation from : Release 21.2.0 · librenms/packer-builds · GitHub
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If you only have root account on your Proxmox host you need to add another user in order to upload the OVA to the Proxmox host so log to your host with ssh and create a new user: (XXXXXX is your new username)
adduser XXXXXX
usermod -a -G root XXXXXX -
It’s a good time to install sudo if it is missing:
apt install sudo -
Allow this user to use sudo (always useful) :
visudo
add line :
XXXXXX ALL=(ALL) NOPASSWD:ALL -
Connect to your Proxmox host with an sftp client from your workstation to upload the OVA
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Login to your proxmox host with ssh and move the OVA
mv /home/XXXXXX/.ova /tmp* -
Decompress image :
tar xvf /tmp/librenms-ubuntu-20.04-amd64.ova -
Create a VM from the GUI without any disks (you will use the downloaded one after)
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Import the decompressed disk to that vm from your ssh console: (YYYY is the vm id you just created)
qm importdisk YYYY librenms-ubuntu-20.04-amd64-disk001.vmdk local -format qcow2 -
Return to the Proxmox gui and attach the disk to the VM (double click the unused disk)
-Click add
-Go in the VM “Option Tab” and double click “Boot Order”:
-Select the disk you just added and attached -
Boot the VM. If there is no login prompt, press enter.
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Login with the cred you get from : Virtual machines - LibreNMS Docs
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It is a good time to change that password
passwd -
Now you probably have no network at this point so lets list the interfaces, check with :
ip a
Remember the interface name (something like ens18)
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Edit netplan:
sudo vi /etc/netplan/00-installer-config.yaml
Under “ethernet:” rename the interface to what you got from IP A (ens18….)
Save and quit -
Apply update with :
netplan apply
reboot
check your ip and access the librenms from a web browser 
Remember the librenms password is not the same from ssh and the web gui.
Hope that help