modifyrepo: 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/modifyrepo - from package: createrepo

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

On Debian possible default full paths are:

/usr/bin/modifyrepo - from package: createrepo

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

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