rkdcop: 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/rkdcop - from package: rekall

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

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