API key not valid. Please pass a valid API key

curl 'https://www.googleapis.com/youtube/v3/subscriptions?key=$KEY'
{ "error": { "code": 400, "message": "API key not valid. Please pass a valid API key.", "errors": [ { "message": "API key not valid. Please pass a valid API key.", "domain": "global", "reason": "badRequest" } ], "status": "INVALID_ARGUMENT", "details": [ { "@type": "type.googleapis.com/google.rpc.ErrorInfo", "reason": "API_KEY_INVALID",...