man: 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/man - from package: man

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

On Debian possible default full paths are:

/usr/bin/man - from package: man-db

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

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