"I can't" is the contraction of "I cannot" used in everyday English. It expresses inability, lack of permission, refusal, or emotional overwhelm. It tells the listener that something is not possible or acceptable for the speaker.
I've never met an animal I didn't like, and I can't say the same thing about people. Doris Day
I like fashion to go down to the street, but I can't accept that it should originate there. Coco Chanel
I can't understand why people are frightened of new ideas. I'm frightened of the old ones. John Cage
Who you are is speaking so loudly that I can't hear what you're saying. Ralph Waldo Emerson
I can accept the failure, but I can't accept not trying. Michael Jordan
If I see you often, I can't call you a visitor. African Proverb