Is the employee active in Human Resources? If not, this will need to be set.
In the user record, is the “Staff Portal Access Allowed” checked? If not, this will need to be set.
Is the user in only one group and does that group have the correct permissions? If they are in more than one group, this can cause conflicts, best to only have one. Permissions can vary but they should have the permissions required for their job.
Is the user listed as an employee? This should display on the page under roles. If they are not, then double check that there are not duplicate person records. Sometimes if you have not searched for a person before adding it, you can end up with duplicates. If the employee and the user are not connected to the same person with the same campus id, this will not work. You will need to delete the employee or user and reconnect them to the same person.
There are several things to check.