On my MX Linux, package not found; maybe the command works on Ubuntu or it needs some repositories to enable first.
You can go the snap route too, if you’re into that:
https://snapcraft.io/install/openra/debian
Enable snapd
On Debian 9 (Stretch) and newer, snap can be installed directly from the command line:
sudo apt update
sudo apt install snapd
sudo snap install core
Install OpenRA
To install OpenRA, simply use the following command:
sudo snap install openra