S6720 - Can't assign an IP address to the sub-interface

what's your device type and software version

>display version
Huawei Versatile Routing Platform Software
VRP (R) software, Version 5.160 (S6720 V200R008C00SPC500)
Copyright (C) 2000-2015 HUAWEI TECH CO., LTD
HUAWEI S6720-30C-EI-24S-AC Routing Switch uptime is 36 weeks, 6 days, 4 hours, 18 minutes

ES5D2S26Q002 0(Master) : uptime is 36 weeks, 6 days, 4 hours, 17 minutes
2048M bytes DDR Memory
446M bytes FLASH
Pcb Version : VER.B
Basic BootROM Version : 0208.0001 Compiled at Oct 13 2015, 14:15:20
BootLoad Version : 0208.0001 Compiled at Oct 24 2015, 02:02:20
CPLD Version : 0107
Software Version : VRP (R) Software, Version 5.160 (V200R008C00SPC500)
CARD1 information
Pcb Version : ES5D21Q04Q01 VER.A
CPLD Version : 0104
PWR1 information
Pcb Version : PWR VER.A
PWR2 information
Pcb Version : PWR VER.B
FAN1 information
Pcb Version : NA

I upgrade to V200R009C00SPC500 and IP command works but IPv6 is not avaliable in sub-interface.

"ip address ip-address { mask | mask-length } [ sub ]" should be configured at a layer 3 interface, so you need to switch the port to layer 3 interface, you can run the command "undo portswitch" to switch to layer 3 interface.