The HTTP401Unauthorized client error response status code indicates that a request was not successful because it lacks valid authentication credentials for the requested resource. This status code is sent with anHTTP... The HTTP401Unauthorizederrorisa status code 401 returned by the server when authentication fails. It means the server has received your request, but it refuses to authorize it. What Does a 401Error Response Look Like? Resolving the HTTP401UnauthorizedError on Your Server. Causes and Possible Solutions for Error401. How authentication and authorization errors differ. 401Unauthorized indicates the client is not authenticated, requiring valid credentials. The 401UnauthorizedError can bea frustrating roadblock when trying to access a website or web application. This comprehensive guide will help you understand what causes this errorand provide practical solutions toresolveitquickly. The funny thing is that when you access your API without a token, with an incorrect token, or even with a token that is not authorized, the error that will return is always the “401unauthorized”. Sometimes it is wrong because it should be “Not authenticated” — I don’t even know who you are. The 401UnauthorizedErrorisanHTTP status code indicating that the server (in this case, FCM) requires valid authentication credentials to process the request, and the credentials provided were missing, invalid, or expired.