Screw up and screw over mean very different things. If you screw up something, then you made a mistake or you did it incorrectly. For example, you can screw up a recipe if you don't cook it right. If you screw over someone, then you did something that is bad for someone else. For example, you can screw over your coworker if you don't come to work, because then they have to do your job.

Thank you, DavidK and Gary! >You can only screw someone over. I'll keep it in mind, thank you!

Exactly - You can't screw something over. You can only screw someone over.

Your first sentence isn’t a correct use of “screw over”. It doesn’t make any sense.