Generally this means the field is inactive or has no data to read. Try using the read last entry api https://api.thingspeak.com/channels/266256/fields/2/last.json but replace it with your channel id and the field you are using for the lamp indicator. You can just enter that in a browser window address field and see if there is a value. If your channel is private, you wil nead a read api key as well. You can check out the doc page on the api for those details. https://www.mathworks.com/help/thingspeak/readlastfieldentry.html
I would also check the channel settings tab of your channel view and make sure the check box next to the fied you are trying to read with the widget is enabled.
