Set up offers the app store as its last step. If you skipped it, or it couldn't be added, searching for Discover finds nothing. Add it this way.
- Open Konsole and paste the first line below (see Paste a line into Konsole). Type your password when asked; it stays invisible as you type. Wait until it finishes.
- Paste the second line. The computer restarts, and Discover is ready, with the apps on Flathub.
sudo /usr/ share/ shuffle/ setup/ shuffle-add appstore systemctl reboot