kxd-add-client-key: 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/kxd-add-client-key - from package: kxd

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

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