dhcp_release: 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/dhcp_release - from package: dnsmasq-utils

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

On Debian possible default full paths are:

/usr/bin/dhcp_release - from package: dnsmasq-utils

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

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