From what I am seeing, Proxmox cannot mount an iSCSI share if it uses CHAP. At least I do not see an option for it in the GUI. I have a mix of shares with CHAP and without CHAP.
You can plug your CHAP value in /etc/iscsi/iscsid.conf , if you have more than one CHAP creds then you will need to establish the session manually. Make sure to mark it for auto-start.