This occurs when the field input specified within the JSON Data node is blank. When paired with an HTTP node, this usually means that the HTTP node successfully completed but didn't return any data. Does the "Get Rows" HTTP node return anything in its specified Output field?
If the Output field is blank, check the results in the HTTP node's Status and Message fields. Most likely, the HTTP node successfully completed but is returning an error message from the endpoint, such as an HTTP 401 Unauthorized. This would result in a blank output that would then impact the JSON Data node.
Status and Message outputs aren't required fields and can be added with the following properties, followed by a node rerun:
