The short answer is that you can not fix this issue. The AA number is programmed in the factory and there is no tool available to restore this in the field. The absence of this number in your BIOS is an indication that corruption has occurred. This could be an indication of a hardware failure (your board is ancient, well past its MTBF date and this is thus not unexpected). Replacing the board is your only option.
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federico128, Scott is right; in this case there not much you can do. This error, as mentioned before, indicated a corruption in your BIOS. You could try with a BIOS recovery; however, this will be under your own risk since it might brick the motherboard.
N.Scott.Pearson thank you for the help.