passenger-memory-stats: 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/sbin/passenger-memory-stats - from package: ruby-passenger

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

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