Typically, this is because they have not been granted access to login to your system. Before a client contact can login, you must make sure they have an email address associated with their account, their access level is not disabled, and you have sent them an invitation to setup their username and password.
Once your client has successfully setup their username and password they will be able to login to your system. If at any time, your client forgets their login credentials, you can have them reset their username and password via the login screen.