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 PLjkpbGd_b6qZN4XOrGCiTAXF8W5eJ6STz belongs to a playlist. Check the FAQ of the plugin or send error messages to support.