Ideally the goal of the iteration should be set by the product owner ( PO), It’s their opportunity to set the agenda and to ensure that everyone understands what the focus of the sprint is. It’s also the responsibility of the scrum team to commit to the goal or if the goal is unobtainable within a single iteration to explain this to the PO. In determining if a goal is achievable the final say is with the scrum team and the PO should be respectful of their opinion (That doesn’t mean that the PO shouldn’t challenge the team however)