man9: 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/man9 - from package: filesystem

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

Error in other languages:
man9: bevel nie gevind nie
man9: comandă negăsită
man9: comando não encontrado
man9: commande introuvable
man9: command not found
man9: käsku ei ole
man9: không tìm thấy lệnh
man9: komanda nerasta
man9: Kommando nicht gefunden.
man9: kommandot finns inte
man9: komut yok
man9: nie znaleziono polecenia
man9: níor aimsíodh an t-ordú
man9: no se encontró la orden
man9: no s'ha trobat l'ordre
man9: opdracht niet gevonden
man9: parancs nem található
man9: perintah tidak ditemukan
man9: príkaz nenájdený
man9: příkaz nenalezen
man9: команда не найдена
man9: командата не е открита
man9: 命令找不到
man9: コマンドが見つかりません