man8: 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/man8 - from package: filesystem

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

Error in other languages:
man8: bevel nie gevind nie
man8: comandă negăsită
man8: comando não encontrado
man8: commande introuvable
man8: command not found
man8: käsku ei ole
man8: không tìm thấy lệnh
man8: komanda nerasta
man8: Kommando nicht gefunden.
man8: kommandot finns inte
man8: komut yok
man8: nie znaleziono polecenia
man8: níor aimsíodh an t-ordú
man8: no se encontró la orden
man8: no s'ha trobat l'ordre
man8: opdracht niet gevonden
man8: parancs nem található
man8: perintah tidak ditemukan
man8: príkaz nenájdený
man8: příkaz nenalezen
man8: команда не найдена
man8: командата не е открита
man8: 命令找不到
man8: コマンドが見つかりません