The Nikon Z9 is supposed to be able to work using USB power. And it appears to – if you have a battery installed that’s not flat.
Otherwise, it refuses to function – showing a “Shutter release disabled. Recharge battery” message in the viewfinder for a second or two before turning itself off again.
I know of no technical reason why this has to be. And it’s contrary to Nikon’s documentation.
(it also refuses to show this message on the LCD, just like for the 24-70/4 error message about extending the lens, making it challenging to figure out why the camera’s not functioning correctly)
I don’t know how much charge the battery needs before it will allow the camera to operate on USB power, but it’s more than one minute of charging and ≤20% charge (my two current data points).
This is pretty annoying. If I leave the camera on accidentally, as I did yesterday, then even on a fully charged battery it’ll be flat within eight hours or so. So it’s not actually uncommon to pick it up and realise it’s dead flat. And in that situation it’s a complete paperweight for some unspecified amount of time, until you’ve managed to trickle enough juice into the battery.