prooftree: 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/prooftree - from package: prooftree

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

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