Maybe that "request" commands is blocked for your user access?
Are you logged in as root user?I checked in JUNOS 17.4 and works fine.
root@jtac-ex9204-re0> show version
Hostname: jtac-ex9204-r2002-re0
Model: ex9204
Junos: 17.4R1.16
JUNOS OS Kernel 64-bit [20171206.f4cad52_builder_stable_11]
JUNOS OS libs [20171206.f4cad52_builder_stable_11]
JUNOS OS runtime [20171206.f4cad52_builder_stable_11]
JUNOS OS time zone information [20171206.f4cad52_builder_stable_11]
JUNOS OS libs compat32 [20171206.f4cad52_builder_stable_11]
JUNOS OS 32-bit compatibility [20171206.f4cad52_builder_stable_11]
JUNOS py extensions [20171219.172921_builder_junos_174_r1]
JUNOS py base [20171219.172921_builder_junos_174_r1]
JUNOS OS crypto [20171206.f4cad52_builder_stable_11]
JUNOS network stack and utilities [20171219.172921_builder_junos_174_r1]
JUNOS libs [20171219.172921_builder_junos_174_r1]
JUNOS libs compat32 [20171219.172921_builder_junos_174_r1]
JUNOS runtime [20171219.172921_builder_junos_174_r1]
root@jtac-ex9204-re0> request system power-off both-routing-engines
warning: Other routing-engine not present
Power Off the system ? [yes,no] (no) yes
Stopping cron.
Waiting for PIDS: 20995.
.
Mar 14 07:36:19 jlaunchd: broker-re (PID 21249) terminate signal 15 sent
Mar 14 07:36:19 jlaunchd: rpcbind-service (PID 21342) terminate signal 15 sent
Mar 14 07:36:19 jlaunchd: overlay-ping-traceroute (PID 21646) terminate signal 15 sent
Mar 14 07:36:23 jlaunchd: iccp-service (PID 21635) exited with status=0 Normal Exit
Waiting (max 60 seconds) for system process `vnlru' to stop... done
Waiting (max 60 seconds) for system process `bufdaemon' to stop... done
Waiting (max 60 seconds) for system process `syncer' to stop...
Syncing disks, vnodes remaining... 0 0 0 0 0 done
All buffers synced.
Uptime: 18d9h51m34s
Khelp module "jsocket" can't unload until its refcount drops from 12 to 0.
acpi0: Powering system off