man6: 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/share/man/bin/man6 - from package: filesystem

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

Error in other languages:
man6: bevel nie gevind nie
man6: comandă negăsită
man6: comando não encontrado
man6: commande introuvable
man6: command not found
man6: käsku ei ole
man6: không tìm thấy lệnh
man6: komanda nerasta
man6: Kommando nicht gefunden.
man6: kommandot finns inte
man6: komut yok
man6: nie znaleziono polecenia
man6: níor aimsíodh an t-ordú
man6: no se encontró la orden
man6: no s'ha trobat l'ordre
man6: opdracht niet gevonden
man6: parancs nem található
man6: perintah tidak ditemukan
man6: príkaz nenájdený
man6: příkaz nenalezen
man6: команда не найдена
man6: командата не е открита
man6: 命令找不到
man6: コマンドが見つかりません