Asus has issued an acknowledgment of serial number issue plaguing several Transformer Prime users while updating to recently released Ice Cream Sandwich update. According to the company, they have successfully reproduced the problem and are working on a fix, which would be out soon. Till then you can try these steps and you might get lucky.
Go to Settings, Backup and Reset, start a Factory Data Reset. Please do this and after the reset let the unit remain active (settings, display, sleep, always on) for 15 minutes with a WiFi connection active so the system can sync with the ASUS servers. Hopefully the system will see the update, if not please go to settings, About Tablet, and then double tap on System Firmware Update
Last week, several users reported cases of not being able to update to Android 4.0 firmware because of “Serial Number Not Found” errors. The problem is however limited to select Prime users, rest of the users were successfully able to update to ICS.