I would gather my purchase receipts and make an appointment with an Apple Genius to see if I could get this serial number issue sorted. https://everymac.com/ultimate-mac-lookup/
Additionally, if the Macs are older, try entering the numbers here at EveryMac https://everymac.com/ultimate-mac-lookup/ and see what info you come up with.
And if you are typing these numbers in manually, be certain that you are not making typos, like entering O for zero. Copy and paste is the way to go.
Unfortunately, there is little that anyone here in the Communities can do. We are but humble users like you and have no direct connection to Apple. And we are told Apple do not participate here.
EDIT - It occurs to me that there is another serial number location to check for each device. In addition to the serial number displayed in About This Mac, you should also find the matching serial number printed on the underside of the MacBook computer and on the underside of the iMac pedestal.