dnsdict6: 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/dnsdict6 - from package: thc-ipv6

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

Error in other languages:
dnsdict6: bevel nie gevind nie
dnsdict6: comandă negăsită
dnsdict6: comando não encontrado
dnsdict6: commande introuvable
dnsdict6: command not found
dnsdict6: käsku ei ole
dnsdict6: không tìm thấy lệnh
dnsdict6: komanda nerasta
dnsdict6: Kommando nicht gefunden.
dnsdict6: kommandot finns inte
dnsdict6: komut yok
dnsdict6: nie znaleziono polecenia
dnsdict6: níor aimsíodh an t-ordú
dnsdict6: no se encontró la orden
dnsdict6: no s'ha trobat l'ordre
dnsdict6: opdracht niet gevonden
dnsdict6: parancs nem található
dnsdict6: perintah tidak ditemukan
dnsdict6: príkaz nenájdený
dnsdict6: příkaz nenalezen
dnsdict6: команда не найдена
dnsdict6: командата не е открита
dnsdict6: 命令找不到
dnsdict6: コマンドが見つかりません