mkvextract: 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/mkvextract - from package: mkvtoolnix

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

On Debian possible default full paths are:

/usr/bin/mkvextract - from package: mkvtoolnix

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

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