mod: 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/mod - from package: monodoc

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

On Debian possible default full paths are:

/usr/bin/mod - from package: monodoc-base

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

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