WooCommerce website does not pass the Basic Authentication header information.
Vend for Mac or PC
Occasionally some servers may not resolve the Authorization header correctly. If you see a "Consumer key is invalid/missing" error when authenticating over SSL, this means you have an issue with your server.
If your website (and WooCommerce) does not allow connections with Authentication Headers in the Vend API. You will need to check with the person who built your website and ask them to allow the Vend API to be used with Authentication Headers.
We send the authentication via "HTTP Basic Auth" as described in WooCommerce's API documentation. We suggest passing this information on to your web developer so they can figure out whatever it is in their server that is blocking this authentication.
It is best to get in touch with a developer or the web hosting company to help you with this issue.
For more information:
Basic Authentication Headers Are Not Passed Through with PHP in CGI Mode: Check Issue #4 -> Solution of this article.