The literal answer to your question is "the product owner". The product owner is responsible for delivering valid stories with acceptance criteria to the team.
However, scrum is merely a framework to help teams learn to be self-directed. The rituals and practices are there to guide a team. At the end of the day, the goal is for a team to work together to solve a problem. Given that, it's the role of the team as a whole to decide how to get the work done if the PO isn't giving proper guidance. It is the role of the scrum master to help the team do that.
If the product owner is not helping by providing valid user stories with usable acceptance criteria, the team should take it upon themselves to create these stories. That might mean bringing in an outside source such as a marketing expert or graphic designer.
The scrum master and product owner should be the ones doing this legwork so that you have proper stories to work from, but again, at the end of the day it is the team that is tasked with producing a product. If the product owner and scrum master can't help you, you have the power to help yourself.