Unavailable Serial Number on late 2012 iMac

When the logic board is replace by an authorized shop, that shop tech should have the tools needed to pair that logic board to the computer and burn the serial number into the firmware (?). When the logic board is user replaced, this doesn't happen.

It's really of little consequence unless one wants to get official support from Apple. Of course doing a logic board swap yourself certainly voids a warranty. In your case the machine is looong out of warranty status.

And, as Mal-S has posted, the serial number of the iMac is printed on the underside of the stand. Any logic board serial number would be printed somewhere on the board itself.

I believe a new board that would be properly installed by a repair tech could be initialized to correctly display the Mac's serial number. This is not a function of reinstalling macOS, so is not the fix for this.

If this is the same part number as was originally assembled into the Mac, then there should be no problem operationally. After all, it's the logic board that controls everything else with instruction from the OS.

Given your concerns about the serial number and the apparent failure of some apps to install, It sounds to me like this board may be faulty. It did come from another, reclaimed Mac, the back story of which you do not know.

Contact the seller and ask if you can exchange for a replacement.

I hope you aren't spending good money on this iMac in hopes of continuing to use it as a front line machine. It is woefully outdated and obsolete.

A search of the group yielded a comment that you only need the serial number when talking to Apple. If you want to find the original serial number its underneath the stand.

You're welcome.

Good luck. 🍀