Notify-send issue

notify-send "Header" "Content" --icon network-idle
su -l user -c 'notify-send "Header" "Content" --icon network-idle'

I’m using the latest updated Qubes OS 4.1 release. Neither of these commands work when called in /rw/config/rc.local or /rw/config/qubes-firewall-user-script.
Please advise.