Error type: “Forbidden”. Error message: “The request is missing a valid API key.” Domain: “global”. Reason: “forbidden”.
Did you added your own Google API key? Look at the help.
Check in YouTube if the id PLhnE51eOCWIW8S0OhZb2ybfnb32jXKGr2 belongs to a playlist. Check the FAQ of the plugin or send error messages to support.