Related article: [Admin] Assign Fax Number
When the user has a fax number assigned to them in the Fax tab (see above related article), you need to ensure that they also have the permission to send faxes on the Gardiant Works profile.
To check this permission, go through the Admin tab and the Users sub-tab, find the person in question and edit their profile, and make sure that the Yes radio button is checked under Can Send Faxes:
Note: If the user has access to multiple firms and you want this user to have access to the fax feature in all firms, please ensure that the permission is checked in the other firm(s), as well.