xemacs-nox-21.5-b31: command not found

Have You tried accessing the binary with the full path?

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/xemacs-nox-21.5-b31 - from package: xemacs-nox

You may install the required package with command yum install {package}
Error in other languages:
xemacs-nox-21.5-b31: bevel nie gevind nie
xemacs-nox-21.5-b31: comandă negăsită
xemacs-nox-21.5-b31: comando não encontrado
xemacs-nox-21.5-b31: commande introuvable
xemacs-nox-21.5-b31: command not found
xemacs-nox-21.5-b31: käsku ei ole
xemacs-nox-21.5-b31: không tìm thấy lệnh
xemacs-nox-21.5-b31: komanda nerasta
xemacs-nox-21.5-b31: Kommando nicht gefunden.
xemacs-nox-21.5-b31: kommandot finns inte
xemacs-nox-21.5-b31: komut yok
xemacs-nox-21.5-b31: nie znaleziono polecenia
xemacs-nox-21.5-b31: níor aimsíodh an t-ordú
xemacs-nox-21.5-b31: no se encontró la orden
xemacs-nox-21.5-b31: no s'ha trobat l'ordre
xemacs-nox-21.5-b31: opdracht niet gevonden
xemacs-nox-21.5-b31: parancs nem található
xemacs-nox-21.5-b31: perintah tidak ditemukan
xemacs-nox-21.5-b31: príkaz nenájdený
xemacs-nox-21.5-b31: příkaz nenalezen
xemacs-nox-21.5-b31: команда не найдена
xemacs-nox-21.5-b31: командата не е открита
xemacs-nox-21.5-b31: 命令找不到
xemacs-nox-21.5-b31: コマンドが見つかりません