In the beginning of project execution, the project manager finds out that a key resource...

 According to the PMBOK Guide, the project manager should update the resource management plan and resource allocation chart to reflect any changes in the availability and assignment of resources. The resource management plan describes how the project resources will be acquired, allocated, managed, and released. The resource allocation chart shows the planned and actual allocation of resources over time. By updating these documents, the project manager can communicate the impact of the resource unavailability to the project team and stakeholders, and plan for alternative solutions to mitigate the schedule risk. References: PMBOK Guide, 6th edition, page 318-319, section 9.1.3.1 Resource Management Plan; page 321, section 9.2.3.1 Resource Allocation Chart.