simplesearch: 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/simplesearch - from package: xapian-core

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

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