Absolutely you can! You just won't get the best performance out of it. Projection screens have optical coatings that enhance their reflective properties. White walls don't. You can certainly use a wall if you want to, and you will get a watchable image. However, compared to the image you'd get with a screen, highlights will not be as brilliant, contrast and colour saturation will be reduced, and (depending largely on the texture of the wall) sharpness will be reduced as well. You will end up with an image that is not as good as your projector is capable of delivering.