Nima,
Sometimes the Serial Number can disappear upon certain replacements to a MacBook, like a logic board replacement. However, I’m mostly concerned about the hardware configuration and I want to help you confirm that your hardware accurately being displayed.
Go into Terminal and run this command:
ioreg -l | awk '/IOPlatformSerialNumber/ { print $4; }'
See if it prints the serial number. Additionally, you can download Coconut Battery which displays the Serial Number. Go [https://www.coconut-flavour.com/coconutbattery/|here] to download it.
[https://forums.macrumors.com/threads/serial-number-unavailable.2159588/|From another forum:] ''“If that doesn't return anything, then you'll likely need to re-serialize your board, which would require Apple's Blank Board serializer, which is normally only available to Apple and authorized repair centers. You may want to try calling around to a few around you just to see if any are willing to do that for you.”''
Let me know if this helps,
-Alex