Error message in the web console: 

"This key is not authorized for use on this domain, if you are the key owner please login to console.temasys.io to manage domains for your key.
Please visit support.temasys.io for further assistance."

API error:

<code>{"success":false,"info":"This key is not authorized for use on this domain, if you are the key owner please login to console.temasys.io to manage domains for your key. Please visit support.temasys.io for further assistance.","error":403}</code>

Error 403 generally occurs when the CORS url is not configured correctly.

Fix: 

Check your Temasys console account to see if the URL from which you are trying to access the app is configured under the CORS URL field. Please make sure that the domain is defined for the key you are using.

Here is a sample screenshot of CORS configuration for the app to be able to run on cdpn.io and localhost. Please note that CORS is configured by default to run on all sub-domains.

Other articles you may be interested in: Can I use wildcards to denote subdomains in the CORS URL?

*For Appian Customers using the Temasys Component, please see: Error: 403