mate-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/mate-system-monitor - from package: mate-system-monitor

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

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