When a project manager is informed during the planning phase that a certain activity cannot commence until another department has implemented the required IT system, the appropriate action is to include this dependency in the project schedule. This ensures that the project timeline reflects all known dependencies and constraints, allowing for accurate planning and resource allocation. Including the dependency in the schedule allows for transparency and sets realistic expectations for project stakeholders. It also facilitates better risk management as it highlights potential delays early in the process, enabling the project team to plan for contingencies.