vp8_simple_encoder: 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/vp8_simple_encoder - from package: libvpx-utils

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

Error in other languages:
vp8_simple_encoder: bevel nie gevind nie
vp8_simple_encoder: comandă negăsită
vp8_simple_encoder: comando não encontrado
vp8_simple_encoder: commande introuvable
vp8_simple_encoder: command not found
vp8_simple_encoder: käsku ei ole
vp8_simple_encoder: không tìm thấy lệnh
vp8_simple_encoder: komanda nerasta
vp8_simple_encoder: Kommando nicht gefunden.
vp8_simple_encoder: kommandot finns inte
vp8_simple_encoder: komut yok
vp8_simple_encoder: nie znaleziono polecenia
vp8_simple_encoder: níor aimsíodh an t-ordú
vp8_simple_encoder: no se encontró la orden
vp8_simple_encoder: no s'ha trobat l'ordre
vp8_simple_encoder: opdracht niet gevonden
vp8_simple_encoder: parancs nem található
vp8_simple_encoder: perintah tidak ditemukan
vp8_simple_encoder: príkaz nenájdený
vp8_simple_encoder: příkaz nenalezen
vp8_simple_encoder: команда не найдена
vp8_simple_encoder: командата не е открита
vp8_simple_encoder: 命令找不到
vp8_simple_encoder: コマンドが見つかりません