First, confirm the Server Rights set up in CONTENTdm Admin, then have the user validate the user name that was provided by logging into the OCLC LDAP account management portal: https://ldap-accounts.oclc.org/
If they are unable to log in, then the user name they have provided is incorrect. Contact OCLC Support and provide the user's email address so the correct user name can be identified.
See additional documentation on setting up CONTENTdm user accounts.