The concept of "not having enough time" to do something is a non-entity: it literally does not exist. When you tell someone that you don't have enough time to do what they want you to do, what you are telling them is that their task is less important than everything else you are doing. There's nothing inherently wrong with this - I get plenty of requests every day that are less important than what I'm doing, both at work and from friends and loved ones - but there are a couple things you need to think about:
- Saying "I don't have the time to do that" means that the request is literally less important than everything else you are doing. Is it really? Can you take just a minute to try to fit it in with everything else going on?
- Most importantly, how would you feel about yourself if you just went ahead and said what you meant, even if you said it as nicely as possible? "Look, I'm really sorry, but your request just isn't important enough for me to get it done today/this week/ever." How would the person with the request feel about you?