Hi veruska,
Sorry the license key is giving you trouble! I’m happy to help get that squared away.
First I’d like to have you clear your browser’s cache and try inputting the license again. If it still doesn’t “stick,” please try another browser. If that is still not working, you’ll need to manually update the PRO plugin to 3.5.1 so that it matches the current version of The Events Calendar (also 3.5.1).
To do this, deactivate and delete the Events Calendar PRO plugin from your Plugins page. You should not lose data, but we recommend that you backup your database first just in case. Then install the new version of the plugin (downloaded from your account’s downloads page) and activate it. That will get you running PRO 3.5.1. Once you are running the latest version of our plugins, I suspect the license key issue will correct itself. Definitely let me know if you take those steps are are still having trouble.
Best,
Leah