Jahanjoon,   We can resend the license if it is available. First I would need you to private message me the svc tag, company name or customer #, and your contact details.  This is in order to verify ownership and system type.   Let me know.    
Hello Dell Support, have seen on the support forum that the Dell Team can assist in recovering lost iDRAC licences. I am not sure as to how it happened, either through an update or other but the iDRAC licence is now gone from my R520 (could be due to a system board change even as the original system board had faults).Happy to share with you the information necessary to hopefully recover the iDRAC licence.  Thank you.
Lost iDRAC Licence PE R520