xxd: 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/xxd - from package: vim-common

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

On Debian possible default full paths are:

/usr/bin/xxd - from package: vim-common

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

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