How to check if you're using the latest version of your theme
To check the version you have installed on your website you can simply go to Appearance-> Themes and click on the installed Themes Details and next to the themes name is the version of it.
Another way you can do this is to search the version in the style.css file of your theme.
The next step is to check what is the last version of the theme you are using and the best way to do that is to go to your Purchase History and see the version number beneath the themes name.
In case you've downloaded the theme from wordpress.org, you can check the version of the theme by accessing the themes page and click on the Preview button.
One of the main reasons on Why is my license key invalid? is due to the fact that you might not be using the latest version of the theme, or your license has expired.