Error type: "Bad Request". Error message: "Bad Request" Domain: "usageLimits". Reason: "keyInvalid".

Did you added your own Google API key? Look at the help.

Check in YouTube if the id PLfrKOWz4om47hTHvDlomTBguFxPmPxmZn belongs to a playlist. Check the FAQ of the plugin or send error messages to support.