dl_poly_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/dl_poly_gui - from package: dl_poly-gui

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

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