You have bought single site license. It is already activated. You can go to that site where it is activated, go to Dashboard > Front End PM > Settings > Licenses and Deactivate. This will allow you to activate license in new site url
I did buy now the 2-5 license to have more than one site also for testing issues, but unfortunately I can not activate anyone of the both sites I am actually running, so I suppose there is a problem with the licennse key or my account …
did you tried to deactivate license? Please remove license key then Save, again input your license key and again save.
If still do not work please give me admin access of your website. Please full backup your website before giving me admin access.