Did you buy directly from Dell? Usually this occurs when you buy from a retailer other than Dell. In which case the system has not been registered with Dell. You can contact Dell tech support and they can help. Also a moderator may jump in here and offer you some further help as well.
Welcome to the Dell Community @mYmY0530
Did you try inserting your "Service Tag" manually???
Thank you for reaching out to the Dell Community Forum.
I had tried verifying the tag with our tag validation team and I am afraid that we could not bring any details.
To assist you further and associate the tag on our records we will need the original purchase invoice. You may try and get in touch with the person who gifted you this if possible.