vp8cx_set_ref: 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/vp8cx_set_ref - from package: libvpx-utils

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

Error in other languages:
vp8cx_set_ref: bevel nie gevind nie
vp8cx_set_ref: comandă negăsită
vp8cx_set_ref: comando não encontrado
vp8cx_set_ref: commande introuvable
vp8cx_set_ref: command not found
vp8cx_set_ref: käsku ei ole
vp8cx_set_ref: không tìm thấy lệnh
vp8cx_set_ref: komanda nerasta
vp8cx_set_ref: Kommando nicht gefunden.
vp8cx_set_ref: kommandot finns inte
vp8cx_set_ref: komut yok
vp8cx_set_ref: nie znaleziono polecenia
vp8cx_set_ref: níor aimsíodh an t-ordú
vp8cx_set_ref: no se encontró la orden
vp8cx_set_ref: no s'ha trobat l'ordre
vp8cx_set_ref: opdracht niet gevonden
vp8cx_set_ref: parancs nem található
vp8cx_set_ref: perintah tidak ditemukan
vp8cx_set_ref: príkaz nenájdený
vp8cx_set_ref: příkaz nenalezen
vp8cx_set_ref: команда не найдена
vp8cx_set_ref: командата не е открита
vp8cx_set_ref: 命令找不到
vp8cx_set_ref: コマンドが見つかりません