tnat64-validateconf: 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

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-validateconf - from package: tnat64

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