plan: 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/plan - from package: plan

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

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