If you enter your serial number and you've entered a lower case L, try the number 1 or vice versa. The same goes for the number 0, if you've entered the letter O, try entering the number instead. If you're still having issues, open a support request at
https://support.presonus.com and send us a picture of the serial number located on the bottom of your interface so a support tech can help get it sorted out for you.