If your key says it is invalid or missing and you're sure you've entered it properly, please try the following:
(1) Be sure your key really is valid. Use the Forgot Key feature to have it re-sent to you and check the expiration date on that email.
(2) Try blanking out the key and saving and then re-entering (paste) the key and saving again.
(3) Check your Firewall and router settings. Be sure nothing is blocking the server application's access to mb3admin.com
(4) Disable or remove any browser add-ins. Some security add-ins (like Privacy Badger) can block our ability to validate the key
If all of the above failed, please send an email to firstname.lastname@example.org (or reply via email if you are already talking to billing support) and include the server log from when you performed the steps in (2.)