proof: 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/proof - from package: alliance

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

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