nmcli: 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/nmcli - from package: NetworkManager

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

On Debian possible default full paths are:

/usr/bin/nmcli - from package: network-manager

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

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