Do you have mem_sleep_default=deep in your kernel command line options?
Check the output of this command in dom0 terminal:
cat /proc/cmdline
If yes then try to remove it from /etc/default/grub and rebuild the GRUB config by running this command in dom0 terminal:
sudo grub2-mkconfig -o /boot/grub2/grub.cfg
Maybe you’ve specified this option but your machine is not supporting S3 sleep.