network_check: 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/lib/plainbox-providers-1/checkbox/bin/network_check - from package: plainbox-provider-checkbox

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

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