herbstclient: 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/herbstclient - from package: herbstluftwm

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

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