Why?

 If you use a fully integrated plugin (Drupal, Magento1, Magento2 or Prestashop), you have to uncheck this setting in the Shipping Manager because the rules to create labels are already defined in the plugin itself. Leaving it checked might give conflict and interfere with the correct functioning of the plugin.

How?

In your Shipping Manager Admin panel, go to General Settings, and uncheck the box next to Webservice v3

Note: Do not forget to "Save configuration"