nm-connection-editor: 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 CentOS possible default full paths are:

/usr/bin/nm-connection-editor - from package: NetworkManager-gnome

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/nm-connection-editor - from package: network-manager-gnome

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

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