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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 PLz4CHFt5_4Qp1zn9yXfz50NRU9Wpq7F_C belongs to a playlist. Check the FAQ of the plugin or send error messages to support.