Hey this is due to the lack of materials, when this happens to me I first try selecting the hair and trying to add a material from product (not the head cap but the hair, check in the scene pane to be sure).
When that doesn't work I look for a setting in the product to make the hair visible in preview and render.
There might be more to it, and I hope it's understandable otherwise try asking in this thread
https://f95zone.to/threads/3d-software-help-and-assistance-ask-away.11702/