xnmap: 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/xnmap - from package: nmap-frontend

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

On Debian possible default full paths are:

/usr/bin/xnmap - from package: zenmap

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

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