The security code is just copying  the numbers/letters that are displayed into the box beside it. Are you sure you are reading it correctly?

Are you using the same email address as the one that you used to order the another company Boxed 767 Captain with? That might be where it is going wrong.

Note. If you are using Google Chrome, it could be worth trying to check-in using Internet Explorer (or Firefox) instead. Chrome has trouble with the Captain Sim site. :(