mkvmerge: 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/mkvmerge - from package: mkvtoolnix

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

On Debian possible default full paths are:

/usr/bin/mkvmerge - from package: mkvtoolnix

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

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