Although the image builder supports a variety of architectures there may still be some which aren't supported. These especially include systems which have a proprietary boot blob, such as the Raspberry Pi boards.
It's still possible to install the system onto these unsupported devices if you need to. First you'll need to ensure that you have *Debian Stretch* installed and can get ssh access to the system. Then either via ssh, or directly on the target device in the case of an old laptop or netbook:
The installation process will then begin. Depending upon the hardware you're installing onto and your internet connection speed it may take quite a while to install.