To finish your flohmarkt setup you'll need to open its setup URL.
You can find the setup URL in flohmarkts logfile in /var/log/APP if you look for entries like this:
2024-05-06 16:30:24 Flohmarkt is not initialized yet. Please go to
2024-05-06 16:30:24 https://YOUR_URL/setup/SECRET
2024-05-06 16:30:24 in order to complete the setup process
(YOUR_URL will reflect the DOMAIN/PATH you chose for installation and SECRET is a random string)