Hello Li Ping Hsu,
NTP is no longer supported on X430 (SummitLite) platforms in EXOS 15.6 or greater.
Please have a look at this article,
Unable to configure NTP on Summit X430
The article above mentions xos0057210, which states the following:
On Summit X430 series switches, NTP configuration is lost and NTP is not supported when upgrading to ExtremeXOS 15.6.