Whilst Ubuntu is derived from Debian (as is TurnKey), it's not binary compatible (while TurnKey is binary compatible with Debian), so unless you know exactly what you are doing, installing software from Ubuntu PPAs on TurnKey (or Debian) is generally considered a bad idea! add-apt-repository is an Ubuntu utility, that whilst is installable and usable on Debian/TurnKey, doesn't follow best practice for setting up repositories, plus makes adding Ubuntu PPAs really easy, so we figure including it on a server distro (where stability should be king) is also a bad idea.
Regardless, if you want to use ZFS on TurnKey (and/or Debian), then I'd recommend looking at the Debian wiki ZFS page. Unfortunately, the Debian wiki can be a bit hit and miss and the info can sometimes be a bit out of date, but it notes that the package you'll want to install is called "zfsutils-linux". The "contrib" repo should be enabled by default, so you should be able to install it like this:
apt update
apt install zfsutils-linux -y
That is quite an old version, but (assuming you are using TurnKey v16.x) then there is a newer version in buster-backports. To enable buster-backports and install the newer backport:
cat > /etc/apt/sources.list.d/debian-backports.list <<EOF
deb http://deb.debian.org/debian buster-backports main
deb http://deb.debian.org/debian buster-backports contrib
#deb http://deb.debian.org/debian buster-backports non-free
EOF
apt update
apt install -t buster-backports zfsutils-linux -y
Installing Ubuntu PPAs on Debian/TurnKey not recommended
Whilst Ubuntu is derived from Debian (as is TurnKey), it's not binary compatible (while TurnKey is binary compatible with Debian), so unless you know exactly what you are doing, installing software from Ubuntu PPAs on TurnKey (or Debian) is generally considered a bad idea! add-apt-repository is an Ubuntu utility, that whilst is installable and usable on Debian/TurnKey, doesn't follow best practice for setting up repositories, plus makes adding Ubuntu PPAs really easy, so we figure including it on a server distro (where stability should be king) is also a bad idea.
Regardless, if you want to use ZFS on TurnKey (and/or Debian), then I'd recommend looking at the Debian wiki ZFS page. Unfortunately, the Debian wiki can be a bit hit and miss and the info can sometimes be a bit out of date, but it notes that the package you'll want to install is called "zfsutils-linux". The "contrib" repo should be enabled by default, so you should be able to install it like this:
That is quite an old version, but (assuming you are using TurnKey v16.x) then there is a newer version in buster-backports. To enable buster-backports and install the newer backport:
Hope that helps.