Thanks for choosing X and sorry for the trouble you are facing.
Although question is related with the error message. But there are couple of queries in the same, let me try to address one at a time:
1) Please make sure to extract the downloaded file and then upload the installable X Theme zip file. You can refer following post to know more:
2) Bundled version of Cornerstone that comes with X Theme does not require purchase code. All you need to do is install X Theme and proceed with the validation.
3) Can you please let us the error message that is popping up? Also please see if there is any error message in browser console. If you spot error message in browser console, kindly let us know as it will us to point you in right direction to resolve the issue.