I can actually just reply here with what I'd reply in a support ticket
There are a few things to do in a situation like this.
First, double check to see you're using the correct code.
Next try running the game in admin mode and entering the license (If you're on a Windows machine, you can skip this step on a Mac).
To run the game in admin mode, right click on the icon and then you'll see an option to 'run as administrator'.
If you go to the directory where OOTP is installed and right click on the OOTP19.exe icon and then go to 'properties' > 'compatibility' you can select the option to 'run this program as administrator' on that screen and then every time you start the game it will automatically run in admin mode.
Sometimes doing that helps the license to stick.
Also, when entering the license, if you've been copy/pasting it, try entering it manually, if you've been manually entering it, try copy/pasting.
If those steps don't work make sure to whitelist OOTP in your firewall and anti-virus programs. Then try turning off your pc and restarting it as well and then try re-entering the license.
Let me know if that helps. If not I'll try to walk you through some more troubleshooting.