@Bikashkc That is a date code on the microSD Card, not a serial number.
Try email support and make sure to send clear high rez pictures of the front and read of the card and packaging.
To make sure that your card is legitimate, take a look at the packaging first; it should either have a tamper-resistant seal or be very clearly printed. Next, inspect the card for a serial or date code thoroughly, you can check the serial number also, on the SanDisk warranty page. If you still have any doubts, call SanDisk support and show them pictures of the card, packing, and receipt. For your next purchase, just be sure to buy from a well-known store, avoid super cheap prices, and check it carefully before you buy it!