This error indicates a problem with your authentication. If all users are experiencing the same error, check with your IT staff to see if they have recently updated a certificate. If so, send OCLC Support your new certificate so it can be loaded. If your current certificate is going to expire soon, send both your current certificate and your new certificate so both can be loaded.
If the error is only affecting some users, it's possible that their information needs to be added via patron load. Or, there might be typos or incorrect information on the user's account. Contact your IT staff or whoever handles patron load at your institution.
If the user information was loaded correctly via patron load but the user(s) are still getting the error, contact OCLC Support with a screenshot of the error and the names and email addresses of the affected users. Please also send the name and email address of a user who is not affected so the accounts can be compared.