How does Scrum work on large complex stories? We cant do this in a month?

The word potentially shippable does not mean that you ship to production every Sprint in Scrum

You can have stories that span multiple sprints, that may not make it to production. This may be part of a larger EPIC story. The goal is to work on it every sprint. We have seen many instances where the product owner decided to ship the story half way through that process, as they see see that what is developed so far is not complete, but is good enough to ship.