rat-ui: 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/rat-ui - from package: rat

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

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