undertaker-linux-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/undertaker-linux-tree - from package: undertaker

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

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