I'm trying to create a project report that shows utilization for both resources and placeholders. Placeholders are showing as "N/A." How can I include them in the report?

Placeholders do not have utilization %. Here's the formula for utilization %:
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​Utilization % = Total Effort divided by Effective Capacity * 100%
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Placeholders do not have Effective Capacity, because they are not actual resources. You can refer to the Total Effort Column (which will include all allocated hours for people and placeholders), and you can compare that against the Effective Capacity column if you want to see Total Effort vs Capacity.