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.
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.