nmtui-connect: 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/nmtui-connect - from package: network-manager

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

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