opendkim: 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 - from package: opendkim

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

On Debian possible default full paths are:

/usr/sbin/opendkim - from package: opendkim

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

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