The complete SN consists of 16 digits + a 4-character verification code. Please take note.
If you can locate the SN in all three positions below, it's a good idea to verify that they match.
Check the outer packaging box
If you still have the machine's outer packaging box, you can find the serial number sticker on its side.

Check the back of the machine
After unboxing, you can find the serial number sticker on the lower portion of the machine's rear panel.

Settings — About in sequence to view the serial number.