Please clarify:
Does this occur with other games too? Is this an actual card removed error (e.g. while moving the 3DS, contact issues) or does it happen right when starting the game every single time and only this game.
If you can eliminate contact issues:
- Copy all your data from the SD to a computer. Reformat the SD to FAT32, 32kb cluster size, if bigger than 32GB and using Windows with guiformat.
- Check you SD Windows, Linux, OS X.
- If you get any errors replace the SD
- If you get no errors copy your files back on the SD, delete the game with Data Management and redownload it from the shop.