If my granny not can remember the password, she always write it on a piece of paper. I keep saying to her that it's very unsafe. Есть тут ошибка?

Да, есть ошибка в вашем предложении. Вместо "not can remember", правильно будет сказать "cannot remember". Таким образом, предложение будет выглядеть: "If my granny cannot remember the password, she always writes it on a piece of paper. I keep telling her that it's very unsafe."