So, you definitely purchased through Fast Spring's store as opposed to Steam?
Did you ever get the game to work or is the .exe file sitting uninstalled from months ago? Or did you download the latest version of the game from here:
https://forums.ootpdevelopments.com/...d.php?t=350771
What message do you get typing in the license key?
Have you tried cutting and pasting the key in to ensure that the error is not a capital I as opposed to a lower case L?
The more info/context the better. thx