trying to reinstall CS 5.5 upgrade. It errors #20 telling me the serial number is wrong, however, it is not wrong. Business is dead in the water, please help.

I am trying to reinstall a CS 5.5 upgrade. It goes all the way through and then gives me error #20 telling me the serial number is wrong, however, it is not wrong. Business is dead in the water, please help. I don't have the funds to upgrade to latest product. There is no phone support.