Why I shouldn't put important passwords inside a password manager?

Hi, I had a lecture about cybersecurity in my school and they said that important passwords(Email, bank account) should not be stored inside a password manager. They also talked about creating a strong password (min 14 characters, capital letters, numbers, special characters) and how writing passwords down on paper is not an option.

If I didn't save important passwords into the password manager while keeping them strong how am I supposed to do that? I am not gonna remember more …

account bank capital characters cybersecurity down email important letters manager numbers password password manager passwords school special writing