The quick solution is to create a new API key.
Also, kindly go to the “YouTube Gallery => Settings” menu from your WordPress Admin Dashboard and make sure the “Development” mode is disabled there.
Technically, you have hit a daily Quota limit by YouTube. The plugin uses the YouTube Data API to get videos from their server. But, the API is not 100% FREE but they have a daily FREE quota limit (10000 UNITS). Still, their FREE API quota limit is sufficient for the majority of websites and mostly the issue occurs only when you use the “search term” option.
Kindly look at this page https://developers.google.com/youtube/v3/determine_quota_cost for more details.
Our plugin uses the “list” method as we only want to display the videos. On the page above, You can see the “search.list” costs 100 UNITS whereas the other “Playlist” & “Channel” resources cost 1 UNIT. So, the search method is the costliest method among all and you will run out of the “API Quota Exceeded” message very soon if you use it extensively. So generally, we do not recommend using the “Search” source type.
If still you would like to use the “Search” option, you can request YouTube to increase your Quota limit by filling out this form https://support.google.com/youtube/contact/yt_api_form?hl=en
Hope this helped you!