highlight-gui: 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/highlight-gui - from package: highlight-gui

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

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