1.Discount amount & Discount percentage can be added as negative value. Lower Limit can be set to 1.
2.N Times Only can be choosen as Negative.
Set the lower limit to 1.
3.Maximum Discount Quantity Can be choosen as Negative.
4.End Date can be entered as previous date compared to start date.

Is there is any reason for this?

Thanks a lot. We will add a validator to that fields.