A glance at the rebate page seems to only rule out P.O. Boxes, APO/FPO/DPO or international address.
Did you correctly copy the serial number, and not include #, commas, spaces or dashes?
Did you attach a copy of the receipt (dated, store-identified sales receipt that clearly shows the item purchased) in the formats jpg, jpeg, png, pdf, in a file size under 5MB?
M&P Rebate v2 | Smith & Wesson