mcelog: 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/mcelog - from package: mcelog

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

On Debian possible default full paths are:

/usr/sbin/mcelog - from package: mcelog

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

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