You need to use a PS/2 keyboard, as it seems Legacy USB is not enabled.
here's the thing though- from the sounds of it, previously you hot-plugged PS/2 and USB keyboards to try to find one that worked.
Try shutting the machine off, plug in a PS/2 keyboard, and then power up.
If you still get keyboard failure, and have a second PS/2 keyboard to try, power down, unplug the current one and plug in the new one.
if it still fails, it's possible that hot-plugging of PS/2 keyboards has destroyed the PS/2 controller or nearby circuitry.