If the Product Backlog is not sufficiently clear at Sprint Planning, the Developers will find it difficult to create a Sprint forecast they are confident they can meet. This is because:

Sprint Planning is an event where the Scrum Team plans for the upcoming Sprint. The purpose of Sprint Planning is to align the entire Scrum Team around a common goal and a plan for delivering an Increment that meets that goal.

The Developers are accountable for creating a Sprint forecast, which is a selection of Product Backlog items that they intend to work on during the Sprint. The Sprint forecast should be realistic, achievable, and valuable.

The Product Owner is accountable for ensuring that the Product Backlog is transparent, visible, and understood by everyone who needs to work on it. They must collaborate with the Developers and provide clarifications, feedback, and guidance on what items are most important and valuable for the product.

If the Product Backlog is not sufficiently clear at Sprint Planning, it means that there are items that are not well defined, ordered, or estimated. This may make it hard for the Developers to understand what they are supposed to build and why. It may also make it hard for them to estimate how much work they can do or how long it will take them to do it. This may result in a poor or inaccurate Sprint forecast that may affect the quality or value of the Increment.

Other options, such as the Product Owner selecting the Sprint Goal for the Scrum Team so that work can begin, nothing in particular happening, the Scrum Master not allowing this to happen or looking for a new Scrum Master and re-starting the Sprint, or Sprint Planning being canceled so refinement can be done first, are not valid answers as they do not reflect what typically happens or what should happen in Scrum.

References:

[Scrum Guide], page 14, section “Sprint Planning”

[Scrum Guide], page 7, section “Developers”

[Scrum Guide], page 6, section “Product Owner”

[Scrum Guide], page 11, section “Product Backlog”