A YouTube Data API key is required from Google API's to search and play music from YouTube. This is free to generate and comes with 10,000 API calls a day. You can apply to increase the API key through the Google API dashboard.
Watch this video here.
You can name your project anything you wish
Ensure you click Enable on the YouTube Data API v3
Select Web Browser from the 'Where will you be accessing the API from' drop down
Your bot is accessing Public Data
Do not restrict your key.