Ubuntu Desktop was not released specifically for the Raspberry Pi during the release cycle of 20.04 (I believe the first specific Desktop image for the Pi was with 20.10 "Groovy Gorilla").
As of now, the latest release (22.04 "Jammy Jellyfish") has a desktop version for Raspberry Pi.
Another option however (documented in the Raspberry Pi server guide) is to first install Ubuntu Server on the Raspberry Pi, and then afterwards installing your desktop environment of choice.
Under step 5, it is recommended to install a lightweight desktop, like Xubuntu or Lubuntu, with one of these commands:
sudo apt install xubuntu-desktop
sudo apt install lubuntu-desktop
It is also possible to install the default Gnome desktop (but this might be resource heavy on the Pi):
sudo apt install ubuntu-desktop
Installing a desktop package over a server image will not convert it into the full desktop installation, since you also need to manually install all applications of your choice. But this may actually be preferable for some, since you then have better control over both which desktop environment and which applications you want on the system.