Hello friend,
When you have your work order task, is your part requirement defined in the related list? Are you defining the part req manually or as part of a template?
Regardless, check here as well:
"To automatically reserve the required parts in agent stockroom, enable the Reserve parts in agent stockroom configuration option on the Field Service Configuration screen.
Note: The required parts must be available in the agent personal stockroom or group stockroom to reserve them for a work order task."
https://docs.servicenow.com/en-US/bundle/sandiego-field-service-management/page/product/planning-and-policy/concept/c_PartRequirements.html
Let me know if that was helpful.
