Running update via CLI:
sms-appliance-release.noarch 10.6.4-3 symrepo_smg
Unable to access repository.
This issue is caused when the date/time of the Messaging Gateway is set to a date/time which is before the official release of the later version.
The solution is to manually set the date and time in the Control Center. See article How to change the date and time manually on a Messaging Gateway for instructions.