px-backup-client-ubuntu

sudo apt update

# installing dependencies
sudo apt install libfuse3-3 qrencode

# linking a newer version to an older version' name
sudo ln -s /usr/lib/x86_64-linux-gnu/libapt-pkg.so.6.0.0 /usr/lib/x86_64-linux-gnu/libapt-pkg.so.5.0.0
sudo ln -s /usr/lib/x86_64-linux-gnu/libapt-pkg.so.6.0 /usr/lib/x86_64-linux-gnu/libapt-pkg.so.5.0

# getting proxmox-backup-client
wget "http://download.proxmox.com/debian/pbs/dists/buster/pbs-no-subscription/binary-amd64/proxmox-backup-client_1.0.6-1_amd64.deb"

# installung bin-utils and extracting .dep-file
sudo apt install binutils
ar x proxmox-backup-client_1.0.6-1_amd64.deb
tar -xJf data.tar.xz

# making the executables usable in your system
sudo cp -a usr/bin /usr/local
sudo cp -a usr/share /usr/local

https://forum.proxmox.com/threads/how-to-install-proxmox-backup-client-under-ubuntu.81371/