Modality is the grammatical expression of the speaker’s attitude or opinion about the possibility, necessity, or certainty of an action or state. Can’t is a modal verb that shows the negation of the verb can. It means that the subject is unable to do something, such as perform an action or demonstrate an ability. It can also be used to express deduction, which is a logical inference based on evidence or reasoning. For example:

She can’t swim. (ability)
He can’t be at home. It’s too early. (deduction)