plot-entropy: 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/plot-entropy - from package: rdd

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

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