clearbuffer-mixed-format: 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/clearbuffer-mixed-format - from package: piglit

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

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