I can't install gnome schedule on Ubuntu 16.04

gnome-schedule is indeed not in the official Ubuntu repositories for Ubuntu 16.04.

However, at https://bugs.launchpad.net/ubuntu/+source/gnome-schedule/+bug/1576060/comments/23, Kartik (kartik-arora95) has kindly compiled a .deb package for version 2.3.0 of Gnome Schedule that is reportedly working for Ubuntu 16.04. Download the .deb package provided at the aforementioned link and then install it using:

sudo dpkg -i gnome-schedule_2.3.0-0ubuntu16.04_amd64.deb