The Cockpit ZFS Manager requires Cockpit version 201 or above. In the Debian Buster repository there’s only cockpit version 188, so you have to use the buster backports repository, which contains cockpit version 223.
# install cockpit
echo "deb http://deb.debian.org/debian buster-backports main" > /etc/apt/sources.list.d/buster-backport.list
apt update
apt-get -t buster-backports install cockpit
# add ZFS manager
git clone https://github.com/optimans/cockpit-zfs-manager.git
cp -r cockpit-zfs-manager/zfs /usr/share/cockpit
# start cockpit
systemctl start cockpit.service
systemctl enable cockpit.service
systemctl status cockpit.service
Now browse to https://ip-address-of-machine:9090 and login.