dndir: command not found

Have You tried accessing the binary with the full path?

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 Debian possible default full paths are:

/usr/bin/dndir - from package: dnet-progs

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