mcrl2-gui: 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/mcrl2-gui - from package: mcrl2

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

Error in other languages:
mcrl2-gui: bevel nie gevind nie
mcrl2-gui: comandă negăsită
mcrl2-gui: comando não encontrado
mcrl2-gui: commande introuvable
mcrl2-gui: command not found
mcrl2-gui: käsku ei ole
mcrl2-gui: không tìm thấy lệnh
mcrl2-gui: komanda nerasta
mcrl2-gui: Kommando nicht gefunden.
mcrl2-gui: kommandot finns inte
mcrl2-gui: komut yok
mcrl2-gui: nie znaleziono polecenia
mcrl2-gui: níor aimsíodh an t-ordú
mcrl2-gui: no se encontró la orden
mcrl2-gui: no s'ha trobat l'ordre
mcrl2-gui: opdracht niet gevonden
mcrl2-gui: parancs nem található
mcrl2-gui: perintah tidak ditemukan
mcrl2-gui: príkaz nenájdený
mcrl2-gui: příkaz nenalezen
mcrl2-gui: команда не найдена
mcrl2-gui: командата не е открита
mcrl2-gui: 命令找不到
mcrl2-gui: コマンドが見つかりません