Every once in awhile I run into an issue where I need a serial number off of an iMac desktop. When I click on about this Mac by serial number it's sometimes blank or it says unavailable. Usually under the pedestal of an iMac desktop there is a hard to read serial number along with a barcode.
However, in the long run it's easier to click on 'about my Mac', it's eaiser to read, and most important without that serial number in the UEFI certain Mac programs like iCloud will not run.
Today I had the no serial number issue, but I had sometime to find a solution.