drivers - Failed to start dGPU off during boot

I got this message "Failed to start dGPU off during boot." inside my logs system (Ubuntu 18.04). This error caused my discrete GPU (Nvidia 940mx) to still run although I have switched into integrated GPU (Intel HD 620). So the battery drain faster.

I tried to figure out the detailed information about this error using,

systemctl status nvidia-prime-boot.service

the result was :

● nvidia-prime-boot.service - dGPU off during boot

   Loaded: loaded (/lib/systemd/system/nvidia-prime-boot.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Tue 2018-07-10 23:42:25 WIB; 27min ago
  Process: 996 ExecStart=/bin/sh -c echo OFF > /sys/kernel/debug/vgaswitcheroo/switch (code=exited, status=2)
 Main PID: 996 (code=exited, status=2)

Jul 10 23:42:28 kharismaPC sh[996]: /bin/sh: 1: cannot create /sys/kernel/debug/vgaswitcheroo/switch: Directory nonexistent
Jul 10 23:42:25 kharismaPC systemd[1]: Starting dGPU off during boot...
Jul 10 23:42:25 kharismaPC systemd[1]: nvidia-prime-boot.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
Jul 10 23:42:25 kharismaPC systemd[1]: nvidia-prime-boot.service: Failed with result 'exit-code'.
Jul 10 23:42:25 kharismaPC systemd[1]: Failed to start dGPU off during boot.

There's something wrong with vgaswitcheroo

Please tell me how to solve this problem

System info :

• Ubuntu 18.04



• Nvidia Proprietary driver 390