systemd-gnome-ask-password-agent: 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/systemd-gnome-ask-password-agent - from package: systemd-ui

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

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