mcrl2i: 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/mcrl2i - from package: mcrl2

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

Error in other languages:
mcrl2i: bevel nie gevind nie
mcrl2i: comandă negăsită
mcrl2i: comando não encontrado
mcrl2i: commande introuvable
mcrl2i: command not found
mcrl2i: käsku ei ole
mcrl2i: không tìm thấy lệnh
mcrl2i: komanda nerasta
mcrl2i: Kommando nicht gefunden.
mcrl2i: kommandot finns inte
mcrl2i: komut yok
mcrl2i: nie znaleziono polecenia
mcrl2i: níor aimsíodh an t-ordú
mcrl2i: no se encontró la orden
mcrl2i: no s'ha trobat l'ordre
mcrl2i: opdracht niet gevonden
mcrl2i: parancs nem található
mcrl2i: perintah tidak ditemukan
mcrl2i: príkaz nenájdený
mcrl2i: příkaz nenalezen
mcrl2i: команда не найдена
mcrl2i: командата не е открита
mcrl2i: 命令找不到
mcrl2i: コマンドが見つかりません