sed: 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:

/bin/sed - from package: sed

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

On Debian possible default full paths are:

/bin/sed - from package: sed
/usr/lib/plan9/bin/sed - from package: 9base

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

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