mkrequest: 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/mkrequest - from package: sscep

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

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