xmodmap: 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/xmodmap - from package: xorg-x11-server-utils

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

On Debian possible default full paths are:

/usr/bin/xmodmap - from package: x11-xserver-utils

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

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