dnssecwalk: 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/dnssecwalk - from package: thc-ipv6

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

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