There are certain types of WiFi that can't be picked up by the DS Lite which prompts you error 51099. For example, you can't use a security protected WEP WiFi. I encountered this problem with my DS also and if you know how to remove the password on your WiFi router, that should solve the problem, however it leaves your network unsecured so I suggest doing that only to receive the event gift.
Nintendo also has a detailed troubleshoot guide for that error which I've provided a link below.
http://www.nintendo.com/consumer/wfc/en_na/ds/results.jsp?error_code=51099&system=DS&locale=en_US
Cheers and good luck with getting Victini!
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