mbuffer: 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/mbuffer - from package: mbuffer

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

On Debian possible default full paths are:

/usr/bin/mbuffer - from package: mbuffer

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

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