inventory_firmware: 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/inventory_firmware - from package: firmware-tools

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

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