Error: "Invalid license key" after activating license
Some customers still see an alert that states that their license key is invalid when trying to activate a license or renewing/upgrading an existing license key.
This issue is sometimes caused by a cached "transient".
Transients are used to store information into the WordPress database with an expiration time, the server load does not increase, and in most scenarios, it helps to speed the site.
However if your WordPress database options table (wp_options) accumulates old, expired transients that didn't get automatically cleared, they could end up bloating your database and even slowing down your website and, in this case, may not allow a license key's status to update.
WooCommerce has a tool that allows you to the Products' Transient Cache and all the expired transients from WordPress. You can access it under WooCommerce > System Status > Tools
WooCommerce has a tool that allows you to the Products' Transient Cache and all the expired transients from WordPress. You can access it under WooCommerce > System Status > Tools
Using this tool, you can attempt to clear expired transients from your Wordpress site and then reattempting your license key activation.
Credit to Rafael Minuesa for the screenshot and more details about how to delete expired transient data in Wordpress on his blog here.