Serial number keeps showing unavailable

OAS94 wrote:

No, it hasn't

Sorry in both of them ("About this Mac" and "System information").

what should that supposed to mean?

I am unclear if you bought this Mac brand new, refurbished or used...(?)

Is your Mac still showing in your list of devices— the serial number should be present:

https://support.apple.com/my-support

You may have to take it in to have Apple flash the board in order to restore your serial number.

Call Apple as posted above and let them sort out your device info.

Has the logic board been replaced?

Is the number not showing in "About this MAc...", System Information, or both?

About System Information on your Mac - Apple Support

OAS94 wrote:

No, it hasn't

Not showing in "About this MAc..."

Showing unavailable only

Update your proof of purchase

Contact Apple Support to update your Apple device information.

https://support.apple.com/enech specs-us/HT202703

Correct the purchase date or expiration date for your ...

https://support.apple.com/en-lamr/HT202696

Well, now I am not sure. By far, the most common cause of a missing serial number in the situations you describe is an incompetent job of changing out the logic board.

Is the serial number on the case bottom?