They both use apt as a package manager, but Ubuntu diverges greatly in tis package offering from Debian. Proxmox installs and runs on Debian, so you'll have to explicitly install some Debian packages to get it to work.
However, what you'll find is that the dependency chain runs straight to crazy town, and that you'll be replacing almost every package on your Ubuntu system with Debian repository packages. Then you get a super broken Ubuntu install that is for the most part Debian.
I can't think of any great reasons to use Ubuntu over Debian for this task anyways. On top of that, Proxmox is a distribution. While it's possible to install all of its relevant packages on an existing server (as I hastily described above), that would be a very silly thing to do.