“Don’t do bad” is basically the worst advice you can ever give anyone.
It’s unhelpful because it lacks specificity in what they’d want you to focus on, is grammatically wonky, and focuses on the negative instead of the positive.
Just as if I tell you not to think of an elephant you’ll still think about an elephant, telling someone to avoid failure is setting them up for it.
Instead, focus on providing specific feedback and spin it in a positive way – or simply praise them. If you’re worried about failure rather than focusing on how to succeed, you’re going to be right.