I remember when I read the book “Prioritizing the Product Backlog” by Roman Pichler. He wrote: “I’ll never forget the day when I suggested to the product manager of a new health-care product to prioritize the use case pile in front of her. She looked at me, her eyes widening, and replied, ‘I can’t. They are all high-priority’”.
Quoting to Karen Martin: “When everything is a priority, nothing is a priority”. I have faced this issue too. I tried to solve it by telling the manager that the team couldn’t be working on five tasks at the same time and asked her what she wanted to obtain first. She didn’t hesitate a second and pointed to one of the user stories. “No doubt,” she said, “this one, definitively”.