hm, that’s strange. That’s not the default behavior of Ubuntu server.
Share the outout of this command:
systemctl list-unit-files | grep suspend
If you see sleep.target or suspend.target enabled, disable them with:
sudo systemctl mask sleep.target suspend.target hibernate.target hybrid-sleep.target
If your goal is for it to just stay awake all the time (which is what most people do for servers), masking those sleep targets is probably the simplest fix.