Hi John,
This error means that you have set the minimum PWM value to something other than 0%. So Mach3 is telling you that you are asking it to set the PWM to a value that is less that the minimum value to it is bumping it up to the minimum PWM value.
My guess is that if you look at the Ports and pins config tab for the spindle (attached image) you will see that the minimum PWM % is no zero.
Cheers,
Peter.