Hello Shikhin,
Thanks for writing in! Over the past few release, we have changed the process of validating X. Instead of the API key, re replaced it with a license key. All you need to do is assign the url to your license and use the license code in the site url your like to validate X theme. You can read about these changes from our changelog (https://theme.co/changelog/).
Hope this helps. Kindly let us know.