I check your account and you only had a license for X and you don’t have a license code for stand-alone cornerstone plugin. X and cornerstone stand-alone plugin have a different license code. If you use the X theme, you can have a built-in cornerstone plugin but if you prefer only the cornerstone plugin, you need to purchase the cornerstone stand-alone plugin license code.
Thanks for sharing the receipt of your purchased cornerstone stand-alone plugin. I went ahead and added the product key in your themeco account and then your cornerstone plugin on your website is now validated.