According to Garmin Support, it's necessary to enable the Ping/Push Notification URL in the Garmin Developer Tools page in order to receive data.
In order to begin getting daily summary data through the Health API, you will need to enable a ping/push notification URL at https://healthapi.garmin.com/tools/endpoints. Setting up these endpoints is detailed in section 5 of the API specification document if you need more information. Additionally, if you need further support, please contact our support team at [email protected].