xeyes: 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/xeyes - from package: xorg-x11-apps

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

On Debian possible default full paths are:

/usr/bin/xeyes - from package: x11-apps

You may install the required package with command apt-get install {package}

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