According to the Project Management Professional (PMP) guidelines, during the planning phase of a project, it is crucial to identify all required skills to ensure that the team can deliver all project outcomes. In this case, the project manager should have identified the need for the specific technical skill during the planning phase.
This would have allowed time to either train an existing team member or hire a new team member with the required skill. By doing this during the planning phase, the project manager could have avoided the issue of not being able to produce a deliverable due to a missing technical skill. References: Project Management Professional (PMP) Certification Handbook, PMBOK Guide 6th Edition, Chapter 9: Project Resource Management.