Workstations are often configured to not boot without their keyboard, because the original PS/2 keyboard design did not support hotplugging.
See section, "Hotplugging" in this Wikipedia article. As a result, for the last quarter century or so, most BIOS implementations for user-facing PC's are configured to halt and wait for a keyboard to be attached if one is not detected, and you may need to go into the BIOS and manually clear a tickbox of some sort to allow it to proceed. I don't have a Z400 so I don't know the correct option for yours, but on HP kit it is sometimes "Set Server Mode" under "Security options". There is also usually an option to disable "Press F1 to Continue" on error, which is the usual thing that you are actually getting stuck on. This is generally my first guess for this kind of problem on a non-server PC, and one of many reasons we encourage the use of true server mainboards.
True servers generally have IPMI (or iLO in HP terminology) interfaces that have a virtual USB attached keyboard on servers, and the BIOS is typically set up to accept that as a keyboard, so this usually isn't a problem for a server board. That's why it's important to describe your system when posting. You still haven't posted an adequate description, because there is both the FreeBSD-based TrueNAS Core and the Linux-based TrueNAS Scale, which each have their own quirks and gotchas with various hardware platforms, and even if you tell us "Z400 workstation", that leaves it up to respondents to have to Google for stuff like "what ethernet chipset does this have."
Failing to provide a good problem summary when you post generally results in poor community participation. Because the possible issue matrix is so large when a poster provides so few facts, many times posts such as your original post go unanswered, because there are so many possibilities. You haven't posted an error message or described what's on the screen, told us whether the machine is pingable, described what kind of ethernet hardware the machine has, etc. Please try to remember that community members, including moderators, are not iX employees, and are here on their own time to help you out. We're pretty good at digging the interesting bits out of a pile of "too much" information, but providing too little causes most people to head on to the next thread.
With such an old system, with the bare minimum of RAM, if it did manage to reboot without a keyboard but then later locked up, you'll probably need to put a monitor on it to determine what happened and why. 8GB RAM is getting just a bit shy on RAM. We expect TrueNAS Core to work fine on that in a basic configuration, while Scale is still a beta product, and that's less of a known quantity. I would suggest using TrueNAS Core. If the host is pingable but the web UI is down, that suggests a middleware failure. If the host is unpingable, then I'd suspect HP may have used a Broadcom ethernet chipset, some of which are known to work poorly with FreeBSD. But with such an old system, component age and stability are also factors that can lead to a system lockup. There are so many directions to go with this, and none of it is helped by the sketchy problem report.