During the UAT client is seeing the delivered product is not having features as expected or features are not working.
Client is frustrated, is demanding features to work and if not will stop the project.
As PM you need immediatelly flag this situation, speak with client to understand all concerns, flag and escalate such situations in the project hierarchy. And when having understanding of the situation analyze the impact and the options. I.e. is it worth to continue and look for the solution or you rather stop because it will bring even more negatives to continue.