If you get that error message from the manual LEF page, then you're probably entering an invalid or incomplete license number. The License database doesn't forget licenses. Even the old time-limited ones I got still return a LEF block after more than 7 years.
Follow Kaushik's advice. The serial number has 16 digits and in most cases starts with the ISO country code. For Mexico, that should be 52.
Important: If you copy the serial number from Excel, make sure that the final digit is the actual digit, as Excel is unable to handle more than 15 digits and usually turns the last one into a 0 (zero). That may also produce an error message like the one you posted.