This error occurs when the Bridge cannot communicate with your WHMCS API correctly. 

To confirm this, enable debug in your WHMCS by navigating to the Bridge Settings page in your WordPress Admin, making sure the "Debug" option is ticked and saving your changes. 

Then, go to the Sync tab and run one/all of your sync functions manually by choosing the relevant sync button. 

After the sync has finished, refresh the bridge settings page and go to the "Debug" tab.

You will likely see an error about your IP address being invalid, in that case - see this article.

Otherwise, you will see an Authentication related error, this is when the admin username and password you have entered into the Bridge Settings page is incorrect, please check the login details and update them if necessary.