installkernel: 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:

/sbin/installkernel - from package: grubby

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

On Debian possible default full paths are:

/sbin/installkernel - from package: debianutils

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

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