Whenever you receive this message it either means that invalid API Credentials were used. Make certain that no added spaces are in your API credentials and that the full signature was copied.
The other option is that your endpoints and your credentials are not matching.
If you use Sandbox credentials and live endpoints then you will receive this message. Same thing if you use live credentials for sandbox endpoints.
Here is a link to another Stack Post with the Same Issue
In the post I have links to the PayPal Developer Documentation. It details going live.