You system's first boot on its own power is what electrical engineers call the „smoke test“.

Select the ``Reboot the System'' menu item which will halt the system because rebooting is not supported on S/390 in this case. You then need to IPL GNU/Linux from the DASD which you selected for the root filesystem during the first steps of the installation.

If you are booting directly into Debian, and the system doesn't start up, either use your original installation boot media (for instance, the rescue floppy), or insert the custom boot floppy if you created one, and reset your system. If you are not using the custom boot floppy, you will probably need to add some boot arguments. If booting with the rescue floppy or similar technique, you need to specify rescue root=root, where root is your root partition, such as /dev/sda1.