Hibernate not working in 18.04

When I use the command sudo systemctl hibernate, the system seems to go to hibernation, but when restarted, it is a fresh start without any of the previous windows that were left open.

And when I tried sudo pm-hibernate it says sudo: pm-hibernate: command not found.

Here is my drive configuration if that helps:

/dev/sda = SSD with Windows 10 only (Windows C drive and the reserved partition).

/dev/sdb = HDD with NTFS and EXT4 partitions containing regular data only, no OS here currently.

/dev/sdc = SSD (GPT partition) with Ubuntu 18.04 (/ partition), swap area and a shared NTFS game drive.

GRUB is installed on /dev/sda.

EDIT: I created a new parition table on /dev/sdc using msdos instead of GPT. Then I reinstalled Ubuntu and installed GRUB on /dev/sdc. After these changes, sudo hibernate seems to be working.

ANOTHER EDIT: Because sudo hibernate seems to go to hibernation without locking the screen (i.e. anyone could turn on the PC and automatically be logged in), I added this line to /etc/sudoers (cln is my username):

cln ALL=NOPASSWD: /usr/sbin/hibernate

and made this executable script:

#!/bin/bash

xdg-screensaver lock
sudo hibernate

Now I am able to just double-click on this script, select Run, and directly go to hibernation while also locking the screen. Much more comfortable.