You need to do 6 steps, that are:
1. Install phpMyAdmin
2. Create Symbolic Link
3. Test phpMyAdmin
4. Create MySQL Superuser
5. Obscure phpMyAdmin URL (Recommended)
6. Secure phpMyAdmin (Recommended)

Follow the Steps on this tutorial (it's updated and working):
https://devanswers.co/install-secure-phpmyadmin-nginx-ubuntu-20-04/