Wall must be smooth Make sure that your wall is as smooth as possible before you start.
Wall must be dry Make sure any fresh paint has dried for at least 24 hours prior to application.
Wall must be skimmed If you happen to have a textured wall, you're gonna need to skim it with drywall mud prior to your whiteboard application.
Wall must be dust-free If your wall is mostly smooth but you have a little bit of texture, just use 220 grit sandpaper. Lightly sand the wall thoroughly, dust it, and then you'll be ready for your application. Remember dust is the enemy to proper adhesion.