Before you can begin field mapping, you need to connect the form instance to Klaviyo. You do that by entering a private API key.
Get API Key
Here are Klaviyo’s instructions for generating a private API key https://help.klaviyo.com/hc/en-us/articles/7423954176283
Connect
Enter the private API key in the indicated input field and click “Connect and Fetch Klaviyo Lists“. If the key is correct, the integration will show as active and you can proceed to mapping your fields.
Note: If you added new lists after initial connection you can use “Refresh Klaviyo Lists” button.