2/ I can’t do it
There are situations where this is true.
But instead of cutting off the discussion which might come as being rude, you could simply say “I can’t do this, but here’s what I can do”
And this is a completely game-changing approach.
It sounds much positive and enthusiastic, showing your willingness to help even though you’d be limited in your own abilities.
It’s often easier to stay in your comfort zone and not to push yourself to deliver more.
This is why great CSMs always go the extra mile rather than sitting back and waiting for things to happen.