synaptic: 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/sbin/synaptic - from package: synaptic

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

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