Try this
HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\Symantec Endpoint Protection\LiveUpdate
AllowManualLiveUpdate 0- means liveupdate button will be greyed out. 1-means it will be available to click.
In the same place you can enable product updates by changing the value of
EnableProductUpdates to 1
For Scheduling and Enabling automatic liveupdates.
HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\Symantec Endpoint Protection\LiveUpdate\Schedule
Change the value of
Enabled to 1 – for Automatic updates.
Ref:
Symantec Endpoint Protection –Few Registry Tweaks..