dhcrelay: 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/sbin/dhcrelay - from package: dhcp

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

On Debian possible default full paths are:

/usr/sbin/dhcrelay - from package: isc-dhcp-relay

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

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