N.L.
My son went through the “I Can’t” phase too about age 3. When he would say it I would respond with “Oh that’s too bad because I don’t know how and I was hoping you could show me”. He would get a little smirk on his face and almost puffed out his chest and would do it. LOL I think it didn’t last long with him because he realized how good it felt to accomplish something, even small things, and he was helping (wink) Mommy learn too =-)
He’s 5 now and he’ll say “I Can’t” when he genuinely doesn’t know how, but I’ll respond with “let me show you how”. If he still says “I can’t” I’ll say “perseverance my love!” and then he does.
He loves that word because he learned it from his Taekwon Do instructor who he totally respects.
Anyway just another idea along with the other great responses you’ve received.
Best of luck!