kupfer-exec: 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/kupfer-exec - from package: kupfer

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

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