The firmware for the above won’t let the z-axis go in the negative. At least I wasn’t able to on my machine. Not sure if it was something I was doing or if it was the firmware.
Correct. You shouldn’t be able to go into the negative. It’s a protection to save punching a dint in your build plate.
If you do a G28 / home, then you’ll be able to move to Z0.
You can however use the Z Offset Wizard to set your probe offset correctly.