I can't right now, but maybe later.
Unfortunately, I've had a few things come up.
I'm trying to focus on finishing off some other things.
Sorry but that isn't my strong suit.
I'm sure you will do fine on your own.
I'm afraid I'm committed to something else.
I can't at that time, but I'm happy to help you with something else later.
I really don't enjoy that kind of activity.
I'm sorry, but I have an emergency to attend to.