Thanks for writing in!
Please check the license is you already have added it in your account. You can manage it in https://theme.co/apex/licenses
And to validate X properly, please do the following:
1] Please login to your APEX 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, and insert the license code.
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
Please let us know how it goes.