nmtui: command not found

Have You tried accessing the binary with the full path?

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 - from package: network-manager

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