I built a machine recently and installed the OS as Ubuntu 18.04 LTS. I found the following site that describes how to place the machine in sleep mode (suspend in Ubuntu): https://www.linuxnov.com/3-methods-to-suspend-ubuntu-18-04-lts-bionic-beaver/.
However, when I suspended the machine, I cannot restart it with a Soft-Off (S5) with the power button. I have to perform a Power Off (G3) by flipping the PSU switch to OFF in order to restart it again.
I only found solutions for hibernation such as creating a swap partition and updating the grub file. These do not apply to enabling the suspend state.
I have been able to trigger the suspend, but the machine automatically restarts after suspending. I found one suggested fix for disabling devices the wake from S3 state, but this was not applicable in my case since all S3 wake devices show disabled.