mann: command not found

Have You tried accessing the binary with the full path?

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/mann - from package: filesystem

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