mb-applet-system-monitor: 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/mb-applet-system-monitor - from package: matchbox-panel

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

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