c99: 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/c99 - from package: gcc

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

Error in other languages:
c99: bevel nie gevind nie
c99: comandă negăsită
c99: comando não encontrado
c99: commande introuvable
c99: command not found
c99: käsku ei ole
c99: không tìm thấy lệnh
c99: komanda nerasta
c99: Kommando nicht gefunden.
c99: kommandot finns inte
c99: komut yok
c99: nie znaleziono polecenia
c99: níor aimsíodh an t-ordú
c99: no se encontró la orden
c99: no s'ha trobat l'ordre
c99: opdracht niet gevonden
c99: parancs nem található
c99: perintah tidak ditemukan
c99: príkaz nenájdený
c99: příkaz nenalezen
c99: команда не найдена
c99: командата не е открита
c99: 命令找不到
c99: コマンドが見つかりません