K.C.
I think how you react depends on her frustration level. If she is just saying "I can't" then you can encourage her, ask her to try, etc. If she is frustrated by a task, it might be better to take a step back and try again later. There are so many new things she is doing at this age, she might feel pressure to do everything the right way. It might also help to point out when you don't do something right to show her that it is okay to do things wrong. It really might help to be "be a goof" for a bit, like put the milk in the pantry and say "Oops, Mommy put the milk in the pantry. That's not where the milk goes!" Show her it is okay to make mistakes.