opendkim-genzone: 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 CentOS possible default full paths are:

/usr/sbin/opendkim-genzone - from package: opendkim

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/opendkim-genzone - from package: opendkim-tools

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

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