We configured linear (open) stacking between ICX-6450 switches.
Once we removed stack cable (SFP) over one switch and again connect it than only this switch is reboot.
I already use hitless enable command.
why this switch is rebooting, I need it become standy/master without rebooting it.
Have you set the same stack priority on the two switches? There is more on hitless stacking and your scenario in this paper
In addition to what Simon has mentioned above, you may also take a look at the hitless stacking section on FI cfg guide. Please click on the link below. There are multiple hitless scenarios based on factors such as number of stk units, priority, etc.