extract_networks6.sh: 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/extract_networks6.sh - from package: thc-ipv6

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

Error in other languages:
extract_networks6.sh: bevel nie gevind nie
extract_networks6.sh: comandă negăsită
extract_networks6.sh: comando não encontrado
extract_networks6.sh: commande introuvable
extract_networks6.sh: command not found
extract_networks6.sh: käsku ei ole
extract_networks6.sh: không tìm thấy lệnh
extract_networks6.sh: komanda nerasta
extract_networks6.sh: Kommando nicht gefunden.
extract_networks6.sh: kommandot finns inte
extract_networks6.sh: komut yok
extract_networks6.sh: nie znaleziono polecenia
extract_networks6.sh: níor aimsíodh an t-ordú
extract_networks6.sh: no se encontró la orden
extract_networks6.sh: no s'ha trobat l'ordre
extract_networks6.sh: opdracht niet gevonden
extract_networks6.sh: parancs nem található
extract_networks6.sh: perintah tidak ditemukan
extract_networks6.sh: príkaz nenájdený
extract_networks6.sh: příkaz nenalezen
extract_networks6.sh: команда не найдена
extract_networks6.sh: командата не е открита
extract_networks6.sh: 命令找不到
extract_networks6.sh: コマンドが見つかりません