readpix-z: 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/lib64/piglit/bin/readpix-z - from package: piglit

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

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