This error usually indicates that you are successfully connecting to your SAN target, but the SAN disk you are trying to access does not exist.
Things you can try:
iscsi:iscsi.example.com::::iqn.1992-01.com.example.iscsi:target
then iPXE will connect to LUN 0, since no LUN is explicitly specified. To instruct iPXE to connect to LUN 1, you would need to use an iSCSI root path such as
iscsi:iscsi.example.com:::1:iqn.1992-01.com.example.iscsi:target ^ \_ LUN goes here