nkf: 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/bin/nkf - from package: nkf

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

On Debian possible default full paths are:

/usr/bin/nkf - from package: nkf

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

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