If you're having errors about lower limit-Z even though you have enough travel and tool length, you might need to set the soft limit boundary lower:
In WinCNC with the machine homed, type L21, and press enter. this will turn off your soft limits (so be careful jogging)
Then lower a known length tool in Z until the bit just touches your CNC bed - below the spoilboard.
Then type G37CAL in WinCNC and press Enter.
It should automatically move to the tool height setter and that will set your new Z soft limit as the CNC bed.
If that doesn't work, would it be possible you could send us a video of what happens in case we're missing something?