Thanks for writing in! Product API key has been deprecated. We are now using a license code which you can assign to your site domain. You can do that in your APEX account (https://theme.co/apex/licenses).
To validate X theme properly, please do the following:
1] Please login to your account and assign the license. You need to assign the license to your site using this url: https://www.example.com/. You have to make sure that url you just assign is the same as the one you have in your settings, Settings > General > Site Address
2] Go back to X > Validation again, and insert the license again.
And please review the changes that we made on how you are going to validated your X theme because we made a big change in the process. Please check it out here: https://theme.co/apex/forum/t/setup-product-validation/55
Now if you are still seeing product API key instead of license code, it just means that you are still using an older version of X. Please have it updated manually. You can download the latest version from your dashboard (https://theme.co/apex/dashboard) and do a manual theme update. Please see "Manual Updates" section in this article: https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62 if you are not sure of how you can do the manual theme update.
Hope this helps.