su-to-root: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/su-to-root - from package: menu
/usr/sbin/su-to-root - from package: menu

You may install the required package with command apt-get install {package}

Error in other languages:
su-to-root: bevel nie gevind nie
su-to-root: comandă negăsită
su-to-root: comando não encontrado
su-to-root: commande introuvable
su-to-root: command not found
su-to-root: käsku ei ole
su-to-root: không tìm thấy lệnh
su-to-root: komanda nerasta
su-to-root: Kommando nicht gefunden.
su-to-root: kommandot finns inte
su-to-root: komut yok
su-to-root: nie znaleziono polecenia
su-to-root: níor aimsíodh an t-ordú
su-to-root: no se encontró la orden
su-to-root: no s'ha trobat l'ordre
su-to-root: opdracht niet gevonden
su-to-root: parancs nem található
su-to-root: perintah tidak ditemukan
su-to-root: príkaz nenájdený
su-to-root: příkaz nenalezen
su-to-root: команда не найдена
su-to-root: командата не е открита
su-to-root: 命令找不到
su-to-root: コマンドが見つかりません