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.
Make sure the printer (fax) is displayed in Devices and Printers, Printer, or Printers and Other Hardware. The printer (fax) is displayed as "EPSON XXXXX (FAX)". If the printer (fax) is not displayed, uninstall and then re-install the FAX Utility. See the following to access Devices and Printers, Printer, or Printers and Other Hardware.
Windows 10
Click on the start button, and then select Windows System > Control Panel > View devices and printers in Hardware and Sound.
Windows 8.1/Windows 8
Select Desktop > Settings > Control Panel > View devices and printers in Hardware and Sound or Hardware.
Windows 7
Click the start button, and select Control Panel > View devices and printers in Hardware and Sound or Hardware.
Windows Vista
Click the start button, and select Control Panel > Printers in Hardware and Sound.
Windows XP
Click the start button, and select Settings > Control Panel > Printers and Other Hardware > Printers and Faxes.
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.