mlyacc: 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/mlyacc - from package: mlton-tools

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

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