need more information, what are you trying to do, what is happening etc...
i will just assume you are trying to connect to a protected wpa wifi, and tell you, you need to temporarly switch to wep or make a open network for the ds to connect
It should be noted that the official Nintendo Wi-Fi Connection service for Nintendo DS and the Nintendo Wii ended years ago. In other words, you cannot do online gaming on the Nintendo DS through official means.