mate-network-admin-pkexec: 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-network-admin-pkexec - from package: mate-system-tools

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

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