pwgen.php: 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/share/simplesamlphp/bin/pwgen.php - from package: simplesamlphp

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

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