dkimsign: 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/dkimsign - from package: python-dkim

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

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