The HyperText Transfer Protocol (HTTP) 401 is generated because of the lack of authentication credentials for the target resource.
In simple words, the user is restricted to access the content because of the lack of credentials the user is providing. Suppose a page is password protected and the user is trying to access the page with a false password then it will show an unauthorized HTTP error to that user.
it can be caused because the user has tried to access the private content and accessed it with false credentials. In this case, the server will show an unauthorized error to that user.

It is a client error status response code that indicates that the request has not been applied because it lacks valid authentication credentials for the target resource. which is sent with a with a WWW-Authenticate the header that contains information on how to authorize correctly.