CLI/GUI Shutdown is only functional on Virtual Appliance. For hardware appliance, just physically power it off.
https://knowledge.broadcom.com/external/article/166972/the-proxysg-appliance-does-not-support-p.html
https://techdocs.broadcom.com/us/en/symantec-security-software/web-and-network-security/proxysg/7-3/overview-hyperv/hyper-v-configure-va/hyper-v-power-off-va.html