readelf: 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/readelf - from package: binutils

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

On Debian possible default full paths are:

/usr/bin/readelf - from package: binutils-multiarch
/usr/bin/readelf - from package: binutils

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

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