The 401 Unauthorized error is an HTTP status code that means the page you were trying to access cannot be loaded until you first log in with a valid user ID and password.
If you have just logged in and received the 401 Unauthorized error, it means that the credentials you entered were invalid for some reason.
Check for errors in the URL. It's possible that the 401 Unauthorized error appeared because the URL was typed incorrectly or the link that was clicked on points to the wrong URL - one that is for authorized users only.
If you're sure the URL is valid, visit the website's main page and look for a link that says Login or Secure Access. Enter your credentials here and then try the page again. If you don't have credentials, follow the instructions provided on the website for setting up an account.
If you're sure the page you're trying to reach shouldn't need authorization, the 401 Unauthorized error message may be a mistake. At that point, it's probably best to contact the webmaster or other website contact and inform them of the problem.