luac5.1: 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/luac5.1 - from package: lua5.1

You may install the required package with command apt-get install {package}
Error in other languages:
luac5.1: bevel nie gevind nie
luac5.1: comandă negăsită
luac5.1: comando não encontrado
luac5.1: commande introuvable
luac5.1: command not found
luac5.1: käsku ei ole
luac5.1: không tìm thấy lệnh
luac5.1: komanda nerasta
luac5.1: Kommando nicht gefunden.
luac5.1: kommandot finns inte
luac5.1: komut yok
luac5.1: nie znaleziono polecenia
luac5.1: níor aimsíodh an t-ordú
luac5.1: no se encontró la orden
luac5.1: no s'ha trobat l'ordre
luac5.1: opdracht niet gevonden
luac5.1: parancs nem található
luac5.1: perintah tidak ditemukan
luac5.1: príkaz nenájdený
luac5.1: příkaz nenalezen
luac5.1: команда не найдена
luac5.1: командата не е открита
luac5.1: 命令找不到
luac5.1: コマンドが見つかりません