dtdefs: 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/dtdefs - from package: dnssec-tools

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

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