The use of Challenge Handshake Authentication Protocol (CHAP) during installation is not supported. As such, CHAP should only be enabled after installation.
If your system boots through an iBFT device, configure CHAP in the iBFT BIOS/firmware setup screen. Your CHAP settings will then be used in the next boot.
If your system boots through PXE iSCSI, configure CHAP through iscsiadm. After configuring, use mkinitrd to ensure that your CHAP settings are used in the next boot.