You need to have a valid purchase code to be able to register the theme.
To find the purchase code:
1. Go to ThemeForest and log in to your account.
2. In the upper right corner, hover over your username and click on Downloads.
3. Scroll down to Jupiter X, click on Download button, and then click on the “License certificate & purchase code (text)” link.
4. In the text file, take note of the Item Purchase Code.
It is similar to rt76ed7sd-0000-1234-9655-111111111111