synaptic-pkexec: command not found

Have You tried accessing the binary with the full path?

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

[root@server ~]# echo $PATH

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/synaptic-pkexec - from package: synaptic

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