In the Sprint Planning meeting, the Product Owner and the Development Team were unable to...

Cancel the Sprint. Send the entire team to an advanced Scrum training and then start a new Sprint.

Forecast the most likely Product Backlog items to meet the goal and create a Sprint Backlog based on a likely initial design and plan. Once the time-box for the Sprint Planning meeting is over, start the Sprint and continue to analyze, decompose, and create additional functionality during the Sprint.

Continue the Sprint Planning meeting past its time-box until an adequate number of Product Backlog items are well enough understood for the Development Team to make a complete forecast. Then start the Sprint.

Discuss in the upcoming Sprint Retrospective why this happened and what changes will make it less likely to recur.

Ask everyone to take as much time as needed to analyze the Product Backlog first, and then reconvene another Sprint Planning meeting.