I have installed elementary OS previously without issues. I wanted to try the latest issue and get the following issue when i check on terminal and following a couple of google searches:
Found this link in my searches: Error while doing 'sudo apt update': no Release file
I was able to do apt upgrade.
App-center still doesn't work
Update edit appcenter, elementary and patches .list flies - remove any refrence to Hera / Juno to Bionic and save.
Everything works.
Try reboot , some reason this worked for me. I had issue few times after installing apps via terminal
