Turns out it's not even proper Debian... https://support.endlessm.com/hc/en-us/articles/115004795106-Is-Endless-OS-based-on-Debian- https://support.endlessm.com/hc/en-us/articles/115000097483-Why-can-t-I-use-apt-get-dpkg-dnf-or-rpm-commands-