In Agile, changes are expected and are part of the iterative process. The product owner should take the opportunity to coach the stakeholder on agile principles, explaining how continuous feedback, flexibility, and adaptability are core to delivering value in an Agile project. Instead of pushing back on changes, the product owner can help the stakeholder understand that responding to change helps improve the product and better meet user needs. This helps align the stakeholder's expectations with Agile practices and fosters better collaboration throughout the project.

When the team is facing difficulties in estimating the scope of work due to unclear or ambiguous requirements, the product owner should collaborate closely with the client to ensure that the requirements are clear and well-defined. By refining the product backlog with more detailed and clarified requirements, the product owner can help the team generate more accurate estimates and better understand the scope of work.
This is a fundamental step in ensuring the team can properly plan and execute the project on time.