Hello Artem, yes this is natural. It is one way to say that you must enter (type) 6 letters/numbers. I see this a lot when creating passwords, however, occasionally it will just state "must have 6 characters."

Yes, it's fine. It's the formal language of written instructions. In everyday conversation, you'd probably say something like this: "You've got to type in at least six characters".

oh really. weird that even google cannot find the exact phrase. But thanks for the info Jill

This is used a lot.