dns_tree: 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 Debian possible default full paths are:

/usr/bin/dns_tree - from package: dns-browse

You may install the required package with command apt-get install {package}

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