You get this error when searching Youtube sometimes. I believe the reason for this error is the addon uses an API key with a limit on the number of queries for each day. When a lot of users have done queries and have reached the limit, you can no longer search for anything.

One solution is to register an API key to your account and sign into Youtube. However another better one is to use a Smartphone app like Newpipe, which once you’ve found a video you can then play it in Kodi.