To enable the configuration settings,

1. Go to the Magento 2 Admin Panel.
2. On the left navigation bar, click on Stores and then on Configuration.
   
3. On clicking it you will be navigated to the Configuration page.
   In the left panel click on Custom to Configurable. The sections open as shown below:
   
4. Under the settings in Enable option select Yes to start with the converting of the products.
   As soon as you enable it, the products will be imported into the catalog.
5. Click on the Save Config button to save the settings.