Error type: "Bad Request". Error message: "API key not valid. Please pass a valid API key." Domain: "global". Reason: "badRequest".

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

Check in YouTube if the id PLUl4u3cNGP61vHg_yuYXydlzBs6d0KSTL belongs to a playlist. Check the FAQ of the plugin or send error messages to support.