modifyrepo_c: 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_c - from package: createrepo_c

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

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