nmtui-hostname: 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-hostname - from package: network-manager

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

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