I’ve also ran into this issue that seems to happen to few people on the forum.
This looks like a brand new purchase code that hasn’t been added to a Themeco account yet. Login to your existing account or register a new one to continue.
To give some more into, I was initially able to active Cornerstone using this code, but then I bricked the WP install so I did clean install. I’m pretty sure than I used the license again successfully, but at some point as I was working on the website, it again prompted me to insert license and that hasn’t worked.