When using the systemd command with the --user option to install units by the unprivileged user, the environment must have the variables $DBUS_SESSION_BUS_ADDRESS adn $XDG_RUNTIME_DIR defined. In a Debian 13 Trixie system these variables are initialized if you login on a virtual terminal or if you login remotely via ssh. They are not defined if you execute su - username.

Ho to test if a script can run as a systemd user service:

systemd-run --user --scope --unit=test-debug /home/radio/radio/radio-backend