preprocess-2.6: 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/preprocess-2.6 - from package: python2-preprocess

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

Error in other languages:
preprocess-2.6: bevel nie gevind nie
preprocess-2.6: comandă negăsită
preprocess-2.6: comando não encontrado
preprocess-2.6: commande introuvable
preprocess-2.6: command not found
preprocess-2.6: käsku ei ole
preprocess-2.6: không tìm thấy lệnh
preprocess-2.6: komanda nerasta
preprocess-2.6: Kommando nicht gefunden.
preprocess-2.6: kommandot finns inte
preprocess-2.6: komut yok
preprocess-2.6: nie znaleziono polecenia
preprocess-2.6: níor aimsíodh an t-ordú
preprocess-2.6: no se encontró la orden
preprocess-2.6: no s'ha trobat l'ordre
preprocess-2.6: opdracht niet gevonden
preprocess-2.6: parancs nem található
preprocess-2.6: perintah tidak ditemukan
preprocess-2.6: príkaz nenájdený
preprocess-2.6: příkaz nenalezen
preprocess-2.6: команда не найдена
preprocess-2.6: командата не е открита
preprocess-2.6: 命令找不到
preprocess-2.6: コマンドが見つかりません