tnat64: 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/tnat64 - from package: tnat64

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

Error in other languages:
tnat64: bevel nie gevind nie
tnat64: comandă negăsită
tnat64: comando não encontrado
tnat64: commande introuvable
tnat64: command not found
tnat64: käsku ei ole
tnat64: không tìm thấy lệnh
tnat64: komanda nerasta
tnat64: Kommando nicht gefunden.
tnat64: kommandot finns inte
tnat64: komut yok
tnat64: nie znaleziono polecenia
tnat64: níor aimsíodh an t-ordú
tnat64: no se encontró la orden
tnat64: no s'ha trobat l'ordre
tnat64: opdracht niet gevonden
tnat64: parancs nem található
tnat64: perintah tidak ditemukan
tnat64: príkaz nenájdený
tnat64: příkaz nenalezen
tnat64: команда не найдена
tnat64: командата не е открита
tnat64: 命令找不到
tnat64: コマンドが見つかりません