Unlocking the license means you are freeing the license so that it will be transferred to another account.
The validation error you are getting is usually caused by a mismatch of the domain name registered with the license and the domain of the site you are trying to validate.
Here are some things you can try:
If you are working on a new site, try disabling all plugins then validate the theme installation.
Check if you have correctly assigned the domain/url of the site with the license from your licenses page.
In case you are still getting an issue, please check this article:
Hope this helps.