This is mostly a copy&paste of the article about installing Proxmox 5.X on top of Debian Stretch, but with the links and repositories updated to the new Debian Buster and Proxmox 6.X
The default proxmox installation ISO is notably minimalist, and one way to be able to do simple customization and have a little bit more flexibility to for example choose the partition layout or use an encrypted LVM is to first make a basic Debian installation and then upgrade it to a full blown Proxmox Installation.
This process is simple, fast and is described in detail at the official proxmox wiki here
But here is the tl;dr version with a few extras and useful modifications from the original article:
Start by making a minimal installation of Debian 10,ie. at the software selection screen check only “SSH server” and “standard system utilities”.
After installation boot to your new Debian machine and be sure that you can resolve the host-name of your machine, the command bellow must return an IP address that is not ‘127.0.0.1’.
This step is important because Proxmox expect to have a “real”( non localhost) IP or else the installation of the package ‘proxmox-ve’ will fail during post-install.
An easy way to achieve this is by editing the second line of your /etc/hosts file
192.168.15.77 prox4m1.proxmox.com prox4m1
With that out of the way everything is ready to proceed to the installation of proxmox 6.X
# Adding the repository key to our trusted keychain
wget http://download.proxmox.com/debian/proxmox-ve-release-6.x.gpg -O /etc/apt/trusted.gpg.d/proxmox-ve-release-6.x.gpg
# You wanna be sure to always add repositories with the arch that they contain,
# this way if you want to add i386 later it won't break your installation.
echo "deb [arch=amd64] http://download.proxmox.com/debian/pve buster pve-no-subscription" > /etc/apt/sources.list.d/pve-install-repo.list
#Optional components, you better install these so you can build a custom openvswitch network and compile kernel modules
apt install -y openvswitch-switch pve-headers
apt install -y proxmox-ve postfix open-iscsi
If everything went smoothly, you can proceed and remove a few (now)obsolete packages.
#Optionally remove the os-prober and Debian kernel packages as they are not needed anymore.
apt remove -y os-prober linux-image-amd64 linux-image-$CURRENTVERSION
After rebooting to make sure you are running the correct kernel version you can proceed with the rest of the normal proxmox configuration process.