A huge pet peeve of mine is people who use words wrong - there vs their, your vs you're , you get the point. With managers, one of the most egregious errors is "can't" vs "won't".
"Can't" refers to an act that cannot be done due to some restriction. I can't float in the air because gravity restricts it. It isn't a lack of will or desire on my end, there is a physical limitation. Sometimes these limitations are not physical, but those are more rare.
"Won't" refers to a choice being made, regardless of why someone makes that choice. I do not want to go to prison, so I won't shoot someone in premeditated murder. Some will argue that a law or rule makes something a "can't", but that isn't true. I can do the act, I choose not to.
Managers have the habit of saying "can't" when they mean "won't". This is a problem in 2 areas: first, it is a way to avoid making a decision that you think people won't like; second, it makes you look weak.
An example, an employee asks if they can attend a seminar in a few weeks about a topic that is part of her personal growth plan, but doesn't really have a direct connection to her current role. As manager, you can approve it, but choose not to. Many managers would say "I can't do that because of company policy". WRONG. You choose not to. You know if you approved it, odds are no one would notice or if they asked you could say that you approved it. You may get yelled at or criticized, but it would be done. Or you could simply say that you won't approve it because you think it is important to follow policy.
When you say "can't", to this request, the employee thinks you have no power. They start to resent the company or they recall times you bent the rules and start to resent you.
Own your choice - say "won't" because it is accurate and provide your reasoning. Don't try to blame the rules or the company.