nm-online: 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-online - from package: NetworkManager

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

On Debian possible default full paths are:

/usr/bin/nm-online - from package: network-manager

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

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