retree: 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/lib/phylip/bin/retree - from package: phylip

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

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