Great process-related question, Peter.

Out of the box, Repair Orders in Odoo don’t trigger automatic Purchase Orders when a required product isn’t in stock cuz there are too many variables for the system to recognize and might cause unnecessary order management.

However, there are workarounds to achieve this behavior.

1. Enable Reordering Rules:
   Ensure that the product you're adding to the Repair Order has a reordering rule configured. You can do this by going to Inventory > Products, selecting the product, and setting up a reordering rule. This will ensure that when the product stock goes below a certain threshold (even zero), Odoo knows to trigger a Purchase Order. 
   
   Look up Reordering Rules on Odoo Docs.
   
   
2. Adjust the Repair Workflow:
   By default, Repair Orders don’t check stock levels like Sales Orders do. You’ll need to ensure that your Repair Orders interact with inventory in a way that respects stock availability. To do this, consider enabling the “Stockable Product” behavior for the repair process.
   
   Look up stockable products odoo and you'll find a YouTube tutorial video.
   
   
3. Customization with Automated Actions (or Development):
   If the above doesn’t fully meet your needs, a bit of custom work might be required. You can create an Automated Action in Odoo (found under Settings > Technical > Automated Actions) to trigger a Purchase Order when a Repair Order includes a product that isn’t in stock.
   
   Also, a developer can write a small script to extend the Repair module’s behavior, replicating how Sales Orders handle stock shortages but I feel this overkill for this requirement.