It is not true that a Product Owner must create clear and unambiguous acceptance criteria for each Product Backlog item before it may be selected in Sprint Planning. This is because:
* Acceptance criteria are a set of conditions that a Product Backlog item must satisfy to be accepted by the Product Owner or the stakeholders. They are used to verify that the item meets the expected quality and value standards.
* Acceptance criteria are not mandatory or prescribed in Scrum. They are one of the possible techniques that a Product Owner may use to communicate their expectations and requirements to the Developers and the stakeholders. They are not part of the Scrum Guide or the Definition of Done.
* Acceptance criteria do not need to be created before Sprint Planning. They can be created or refined at any time during the product development process, as long as they are clear and agreed upon before the item is considered complete. They can also be changed or updated based on feedback or new insights.
* The only requirement for a Product Backlog item to be selected in Sprint Planning is that it is sufficiently clear and ready for selection. This means that it has enough detail and precision for the Developers to understand what they are supposed to build and why. It also means that it has been refined and ordered by the Product Owner based on value and impact.
References:
* [Scrum Guide], page 14, section "Sprint Planning"
* [Scrum Guide], page 12, section "Product Backlog Refinement"
* [Professional Scrum Product Owner™ Training], page 7, section "Product Backlog Management"