When you connect to home assistant for the first time the onboarding process is started, where you can create a user and set a password.
If you forgot that password you can reset it: https://www.home-assistant.io/docs/locked_out/
Did you also follow the docs?
After dietpi-software has finished and the service starts the first time, please go through the following steps manually:
Run htop and wait until the CPU usage of the homeassistant processes goes down to nearly zero.
Run systemctl restart home-assistant
Run htop and wait until the CPU usage of the homeassistant processes goes down to nearly zero.
Open the HA web UI (see “Access to the web interface” tab). It will again install some Python modules on first access, which can again take a little while. Always check htop if you are unsure, which reveals any currently running Python/pip module install process.
this will be automatically displayed on first attempt to open HA web site. HA will ask automatically to create a new user / password. There is nothing preconfigured on first access.