If you are using IceWM Desktop, you need to add $HOME/.icewm/startup file:

cd /opt/UPDATE && ./PHASE2 &

CAUTION!

DO NOT INSTALL this non-systemd distro because it installs systemd !!!

This is an experimental package, so bugs are possible !!!

Update-Notifier-systemd-experimental.zip

Works fine with IceWM desktop in Linux Mint 21 also