ml-lex: 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/ml-lex - from package: ml-lex
/usr/lib/smlnj/bin/ml-lex - from package: ml-lex

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

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