It’s okay to be busy. And it is definitely okay and preferable to prioritize and not try to do all of the things.
But when someone makes a reasonable request, it takes the same amount of time to respond politely and tell them that you cannot get to it for X period of time as it does to respond with a tone as if you are exasperated they didn’t know that you were so busy and should not have made the request at this point in time.
In fact, the prior might even leave you feeling more relaxed than the latter.
If it is true that they should not be asking you, then let them know, but even that can be done politely. You might even be able to have someone else do it in appropriate situations, ie when you have staff, part of their job is to run interference so you can get good work done.