ptree: 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/ptree - from package: adacontrol

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

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