mdvalidater: 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/mdvalidater - from package: monodoc

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

On Debian possible default full paths are:

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

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

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