MassCalculator: 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/MassCalculator - from package: topp

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

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