Ubuntu comes with a Workrave package. You can install it from the command line with the following command:
sudo apt-get install workrave
If you need a newer version than what is included in your distribution, use the official Workrave PPA.
The Workrave PPA can be enabled by entering these commands into a terminal:
sudo add-apt-repository --yes ppa:rob-caelers/workrave
sudo apt update
sudo apt install workrave
If you have the Ubuntu version of Workrave installed, you need to uninstall that first:
sudo apt remove workrave workrave-data
The Workrave PPA has separate package for each supported desktop environment. You only need to install the one for your desktop environment.
sudo apt install workrave-cinnamon
sudo apt install workrave-gnome
sudo apt install workrave-mate
sudo apt install workrave-unity
sudo apt install workrave-xfce