simpleindex: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/simpleindex - from package: xapian-core

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