force_keyframe: 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/force_keyframe - from package: libvpx-utils

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

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