rmmod: 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:

/sbin/rmmod - from package: module-init-tools

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

On Debian possible default full paths are:

/sbin/rmmod - from package: kmod

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

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