Install mysql through deb file on ubuntu 20.04
You can use
 mysql-server_8.0.21-1ubuntu20.04_amd64.deb

But yoz should follow the howto
Install short Howto

Download latest mysql-apt-config deb from
https://repo.mysql.com/

Install mysql-apt-config deb:
# sudo dpkg -i mysql-apt-config_*.deb

Update the package index:
# sudo apt-get update

Install mysql-server deb package:
# sudo apt-get install mysql-server


ANd if you have problems see https://dev.mysql.com/doc/mysql-installation-excerpt/8.0/en/linux-installation-debian.html