mongotop: 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 CentOS possible default full paths are:

/usr/bin/mongotop - from package: mongodb

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/mongotop - from package: mongodb-clients

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

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