Testing must be done at another department's facility known for low availability, which may affect the schedule. What should the project manager do next?

The project manager should update the risk register. The low availability of the testing facility is an identified uncertainty that, if it occurs, could negatively impact the project schedule. This fits the definition of a risk, which should be documented in the risk register along with its potential impact, probability, and planned responses. Updating the project schedule prematurely would be reactive; the impact hasn't occurred yet, and a risk response might mitigate or avoid it. Updating the issue log is incorrect because this is a potential future event, not a current problem or issue that has already materialized. Updating the lessons learned register is also incorrect, as this is a forward-looking situation, not an experience to be documented for future reference.