How to change battery low for ups in nut server

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From the error I guess the user has no admin privileges, and a new user might need to be added (with a custom template) to upsd.users file.

EDIT: here is an untested example. Some steps could be missing (see nut manual, for example to set tcp-wrappers edit hosts.allow/deny, restarting service…), and the password could be randomly generated with a script:

mkdir -p /etc/e-smith/templates-custom/etc/ups/upsd.users/
cat << EOF > /etc/e-smith/templates-custom/etc/ups/upsd.users/21admin

[admin]
    password = supersecretpassword
    actions = SET
    instcmds = ALL
EOF

signal-event nethserver-nut-update

More info:
https://networkupstools.org/docs/man/upsd.users.html
https://networkupstools.org/docs/user-manual.chunked/ar01s09.html

NUT administrative user definitions should be used in conjunction with TCP Wrappers.