glxspheres64: 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/glxspheres64 - from package: VirtualGL

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

Error in other languages:
glxspheres64: bevel nie gevind nie
glxspheres64: comandă negăsită
glxspheres64: comando não encontrado
glxspheres64: commande introuvable
glxspheres64: command not found
glxspheres64: käsku ei ole
glxspheres64: không tìm thấy lệnh
glxspheres64: komanda nerasta
glxspheres64: Kommando nicht gefunden.
glxspheres64: kommandot finns inte
glxspheres64: komut yok
glxspheres64: nie znaleziono polecenia
glxspheres64: níor aimsíodh an t-ordú
glxspheres64: no se encontró la orden
glxspheres64: no s'ha trobat l'ordre
glxspheres64: opdracht niet gevonden
glxspheres64: parancs nem található
glxspheres64: perintah tidak ditemukan
glxspheres64: príkaz nenájdený
glxspheres64: příkaz nenalezen
glxspheres64: команда не найдена
glxspheres64: командата не е открита
glxspheres64: 命令找不到
glxspheres64: コマンドが見つかりません