ndpexhaust26: 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/ndpexhaust26 - from package: thc-ipv6

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

Error in other languages:
ndpexhaust26: bevel nie gevind nie
ndpexhaust26: comandă negăsită
ndpexhaust26: comando não encontrado
ndpexhaust26: commande introuvable
ndpexhaust26: command not found
ndpexhaust26: käsku ei ole
ndpexhaust26: không tìm thấy lệnh
ndpexhaust26: komanda nerasta
ndpexhaust26: Kommando nicht gefunden.
ndpexhaust26: kommandot finns inte
ndpexhaust26: komut yok
ndpexhaust26: nie znaleziono polecenia
ndpexhaust26: níor aimsíodh an t-ordú
ndpexhaust26: no se encontró la orden
ndpexhaust26: no s'ha trobat l'ordre
ndpexhaust26: opdracht niet gevonden
ndpexhaust26: parancs nem található
ndpexhaust26: perintah tidak ditemukan
ndpexhaust26: príkaz nenájdený
ndpexhaust26: příkaz nenalezen
ndpexhaust26: команда не найдена
ndpexhaust26: командата не е открита
ndpexhaust26: 命令找不到
ndpexhaust26: コマンドが見つかりません