If you want to play online or download titles from the eShop, you first need to connect your 3DS XL to a WiFi network.
From the HOME menu, tap System Settings. (It's the one shaped like a wrench.)
Choose Internet Settings > Connection Settings > New Connection.
You can set up a maximum of three internet connections on the Nintendo 3DS XL.
Choose Manual Setup. Alternatively, you can select Tutorial if you want to watch a demonstration on setting up WiFi.
Tap Search for an Access Point to locate your WiFi network.
Find the name for your wireless network on the list and then tap it.
If asked, enter your wireless network's password.
Tap OK to save the connection settings.
Choose OK once more to perform a connection test. If everything's fine, you'll receive a prompt letting you know that your Nintendo 3DS XL is connected to WiFi.
From this point forward, as long as your 3DS' WiFi is turned on and you're within range of an approved access point, the 3DS will connect to the network automatically.
IF ALL THE SLOTS ARE TOOK CLICK ON A NON NEEDED ONE AND SELECT REMOVE/DELETE!!!
commented May 4 by Codace
Make sure your game cartridge is not damaged and that the place on your Ds where you put in the cartridge is not damaged as well.
Make sure your game is updated.
I'm almost sure that there is something wrong with the cartridge or ds. If all else fails you can send it to Nintendo