The most recent version of ubuntu server that i can find mongodb packages for is 18.04 (bionic).
However, i just tested installing this package on Ubuntu 20.04 (Focal) and i didn’t get any complaints/errors and appears to have worked. Your mileage may vary.
lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.4 LTS
Release: 20.04
Codename: focal
curl -fsSL https://pgp.mongodb.com/server-4.2.asc | \
sudo gpg -o /usr/share/keyrings/mongodb-server-4.2.gpg \
--dearmor
echo "deb [ arch=amd64,arm64 signed-by=/usr/share/keyrings/mongodb-server-4.2.gpg ] https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/4.2 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-4.2.list
sudo apt-get update
sudo apt-get install -y mongodb-org
mongo --eval "db.version()" --quiet
4.2.24