Update : I have tried the method from N0rbert and it work for me on Ubuntu 21.04.
In Ubuntu 21.04, I've installed GeoDa in a VirtualBox machine from an Ubuntu 20.04.3 ISO.
Another solution might be to create a USB key (Live or persistent) with Ubuntu 20.04.3 ISO. I've tested it with a live USB (you can install software but the configuration is reset when you reboot). I find it great for tests if I write the steps.
The following commands allow me to install GeoDa :
sudo add-apt-repository universe
sudo apt update
sudo apt-get install freeglut3 libpq5 libgdal26
Then I've clicked on geoda_1.18-1focal1_amd64.deb for installation and search for GeoDa in Applications.

Note : I'm an user of other Linux distributions. I've also installed GeoDa with Linux Mint 20.02 (it is based on Ubuntu 20.04 LTS). I don't remember if the steps are the same. It was just a test.
I had no luck with gdebi to install directly on Ubuntu 21.04.
sudo gdebi geoda_1.18-1focal1_amd64.deb
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Reading state information... Done
This package is uninstallable
Dependency is not satisfiable: libgdal26