If your Mautic doesn’t show up in the API Credentials section inside the menu then please make sure that API is enabled in your mautic account. The same can be achieved from Settings > Configuration > API Settings. Please mark the API enabled checkbox to “YES” and your Mautic account will be enabled for API.
You can also visit the URL for setup the mautic  and get the connection key
(https://docs.makewebbetter.com/mautic-woocommerce-integration-app-setup-guide/)