ndpexhaust6: 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/ndpexhaust6 - from package: thc-ipv6

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

Error in other languages:
ndpexhaust6: bevel nie gevind nie
ndpexhaust6: comandă negăsită
ndpexhaust6: comando não encontrado
ndpexhaust6: commande introuvable
ndpexhaust6: command not found
ndpexhaust6: käsku ei ole
ndpexhaust6: không tìm thấy lệnh
ndpexhaust6: komanda nerasta
ndpexhaust6: Kommando nicht gefunden.
ndpexhaust6: kommandot finns inte
ndpexhaust6: komut yok
ndpexhaust6: nie znaleziono polecenia
ndpexhaust6: níor aimsíodh an t-ordú
ndpexhaust6: no se encontró la orden
ndpexhaust6: no s'ha trobat l'ordre
ndpexhaust6: opdracht niet gevonden
ndpexhaust6: parancs nem található
ndpexhaust6: perintah tidak ditemukan
ndpexhaust6: príkaz nenájdený
ndpexhaust6: příkaz nenalezen
ndpexhaust6: команда не найдена
ndpexhaust6: командата не е открита
ndpexhaust6: 命令找不到
ndpexhaust6: コマンドが見つかりません