yindex: 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/yindex - from package: patchy

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

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