mergerepo: 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/mergerepo - from package: createrepo

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

On Debian possible default full paths are:

/usr/bin/mergerepo - from package: createrepo

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

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