The following causes can be considered.
Solutions
Make sure that the PC-FAX driver have been installed on the computer. The PC-FAX driver is installed with FAX Utility. Follow the steps below to check if it is installed.
Select System Preferences from the Apple menu > Printers & Scanners (or Print & Scan, Print & Fax), and then make sure the printer (fax) is displayed. The printer (fax) is displayed as "FAX XXXX (USB)" or "FAX XXXX (IP)". If the printer (fax) is not displayed, click [+] and then register the printer (fax).
Solutions
Select System Preferences from the Apple menu > Printers & Scanners (or Print & Scan, Print & Fax), and then double-click the printer (fax). If the printer is paused, click Resume (or Resume Printer).
Solutions
Set the user name and password in the printer driver. When sending a fax from a computer when the security function that restrict users from using the printer's fax function is set, user authentication is performed with the user name and password set in the printer driver.