Hi Jenny,
No you don’t need to buy a new license every time you update. If you have changed your URL this message may have appeared or it may indeed be a hiccup on our side – please rest assured however that the operation of the plugin (with the exception of automated updates) will not be affected.
That said, you’ll probably want to clear this up so the best steps to try and wiping the license key from your own WordPress installation (delete it and save the change) then clear it from your account here at tri.be.
With that done you should have a “clean slate” and can try punching it in once more.
Hope that helps, but let me know if not.