Problem:
Customer was getting the error 'Unable to register: site key not matching' while trying to update the WPML plugins.

Solution:
If you have this issue, follow the steps below to re-register Toolset to be able to update Toolset as it seems for some reason the key you use is not valid:

- Make sure that you are logged in toolset.com.
- Go to "WordPress Dashboard > Plugins > Add New > Commercial (Tab)".
- Click the "Unregister Toolset" link.
- Click the "Register Toolset" at the right section of the screen.
- Click the "Get a key for this site" link .
- A new page will open up and the auto-generated key will show in pop-up.
- Copy the key and paste it into the "Site Key" input box annotated in the screenshot below.
- Click the "OK" button.
- Go to wp-admin > Dashboard -> Updates
- Click on check again

Relevant Documentation:
https://toolset.com/faq/how-to-install-and-register-toolset/