mdoc-update: 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/mdoc-update - from package: monodoc

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

On Debian possible default full paths are:

/usr/bin/mdoc-update - from package: monodoc-base

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

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