This is probably one of the questions I get asked the most "Can I apply decals to textured walls?" The short answer is, probably not, but it really does depend on how much texture there is on the wall to begin with.
If it looks like this, it's extremely unlikely that any decal would stick, even with the best vinyl on the market, it's just not a viable surface.
If there is only slight bumps, then you can, in theory. If you use a hairdryer to heat the decal while the application tape is still on the wall and then go over the whole thing with a tennis ball, rolling and pressing the vinyl into any lumpy areas. Doing this while it is warm will give the vinyl more flexibility and allow the tennis ball to squish the vinyl into any dips. This isn't a guarantee that the vinyl will stick, but it will give it the best chance. Then leave the application tape on and allow it to all cool. This is not going to be a quick and easy install, it's gonna take some time and patience to make it work.
Ultimately, we'd advise against applying decals to any walls that are lumpity bumpity, but if you are in any doubt, we'll always happily offer a test decal if you get in touch first.