Override total discount

HI ,

I have created a sales order and i wanted to set a total discount % and override total discount,

however, when i enter the total discount % and check the total discount override checkbox, a message box popped out as showed in the attachment.
my question is no matter what i enter, i cannot check the total discount override checkbox.

please advice

Can you please elaborate your question?

I have checked the total discount % check box (as shown in the snapshop above) but when i return to the SO, the total discount override checkbox will unchecked automatically.

Any ideas how to solve it? I am using AX2012