inverse_lookup6: 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/inverse_lookup6 - from package: thc-ipv6

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

Error in other languages:
inverse_lookup6: bevel nie gevind nie
inverse_lookup6: comandă negăsită
inverse_lookup6: comando não encontrado
inverse_lookup6: commande introuvable
inverse_lookup6: command not found
inverse_lookup6: käsku ei ole
inverse_lookup6: không tìm thấy lệnh
inverse_lookup6: komanda nerasta
inverse_lookup6: Kommando nicht gefunden.
inverse_lookup6: kommandot finns inte
inverse_lookup6: komut yok
inverse_lookup6: nie znaleziono polecenia
inverse_lookup6: níor aimsíodh an t-ordú
inverse_lookup6: no se encontró la orden
inverse_lookup6: no s'ha trobat l'ordre
inverse_lookup6: opdracht niet gevonden
inverse_lookup6: parancs nem található
inverse_lookup6: perintah tidak ditemukan
inverse_lookup6: príkaz nenájdený
inverse_lookup6: příkaz nenalezen
inverse_lookup6: команда не найдена
inverse_lookup6: командата не е открита
inverse_lookup6: 命令找不到
inverse_lookup6: コマンドが見つかりません