gst-device-monitor-1.0: 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/gst-device-monitor-1.0 - from package: gstreamer1.0-plugins-base-apps

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

Error in other languages:
gst-device-monitor-1.0: bevel nie gevind nie
gst-device-monitor-1.0: comandă negăsită
gst-device-monitor-1.0: comando não encontrado
gst-device-monitor-1.0: commande introuvable
gst-device-monitor-1.0: command not found
gst-device-monitor-1.0: käsku ei ole
gst-device-monitor-1.0: không tìm thấy lệnh
gst-device-monitor-1.0: komanda nerasta
gst-device-monitor-1.0: Kommando nicht gefunden.
gst-device-monitor-1.0: kommandot finns inte
gst-device-monitor-1.0: komut yok
gst-device-monitor-1.0: nie znaleziono polecenia
gst-device-monitor-1.0: níor aimsíodh an t-ordú
gst-device-monitor-1.0: no se encontró la orden
gst-device-monitor-1.0: no s'ha trobat l'ordre
gst-device-monitor-1.0: opdracht niet gevonden
gst-device-monitor-1.0: parancs nem található
gst-device-monitor-1.0: perintah tidak ditemukan
gst-device-monitor-1.0: príkaz nenájdený
gst-device-monitor-1.0: příkaz nenalezen
gst-device-monitor-1.0: команда не найдена
gst-device-monitor-1.0: командата не е открита
gst-device-monitor-1.0: 命令找不到
gst-device-monitor-1.0: コマンドが見つかりません