How do I repair damage before painting?

Paint it with Kilz before painting.

Use a mask and lightly sand the bumps out. Then kilt before painting.

Also paint with flat paint. Helps hide imperfections.

I would just use a Texture Paint on that. It is much easier than attempting to repair the damage from a Moron.

If your entire room is like this, I agree to use a textured paint, but if you are willing to spend the time and effort, this can be made smooth. I know because I have done it on multiple houses. First, get a 5-in-1 tool. https://www.lowes.com/pd/Warner-Pro-Grip-3-25-in-Paint-Multi-Tool/3364386

This is the best thing for scraping rough places off of your wall. Forget sanding, you will be there all year. Go around the room ad scrape every rough spot. It is not hard, just time consuming. When you think it is smooth, use your hand (not your eyes) and feel the spot. If it feels smooth, it is smooth. When you have done this everywhere, then use a sanding block to go over any areas that have a tiny ridge. Again, use your fingers not your eyes to determine whether your wall is smooth. What looks smooth to your eye is not always so. Then use spackle to fill in any dings or gouges. Smooth these as much as you can with your spackling tool. Try not to leave any excess spackle. It is better to add a second or third coat of spackle than to put on one big glob. Lightly sand between each coat of spackle. If you do this right, you will have almost no sanding to do. Again, use your fingers to feel whether the edges are smooth. When you have completed this step, run your hand over all the areas that have been scraped and/or spackled to see if you missed any spots. Then prime the entire wall and paint. This will give you a beautiful smooth, lovely paint job. This may seem daunting, but you don't have to do it all in one day. The paint job will probably be there for several years, so it is worth taking the time to do it right. Best of luck.