nfreplay: 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/nfreplay - from package: nfdump

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

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