This is my product configuration:

• Product Type: Service
• Create on Order: Nothing
• Invoicing Policy: Prepaid/Fixed Price

Classic issue : this is almost always a product invoicing policy mismatch. Here's the exact fix:

1. Go to **Field Service → Configuration → Settings** and make sure "Timesheets" and/or "Products on Tasks" are enabled under the Invoicing section.

2. Then check the product itself: **Inventory → Products → [your service product]** → Sales tab → make sure "Invoicing Policy" is set to **"Timesheets"** (if billing by time) or **"Based on Delivered Quantity (Manual)"** (if billing by materials/fixed task).

3. On the Field Service task itself, confirm the **Sales Order** is linked (there's a "Sales" smart button at the top). If there's no linked SO, Odoo has nothing to pull invoice lines from.

4. Finally: go to the linked Sales Order → confirm it's in "Sales Order" status (not Draft/Quotation). You can't invoice from a draft SO.

I've configured this for 100+ service businesses — the most common culprit is step 2: someone set the product to "Ordered Quantities" which requires a manually entered delivered qty on the SO line. Switch it to "Timesheets" or set the delivered qty manually on the SO line, then try invoicing again.

If you're still stuck after this, share what version you're on and whether you're using Field Service standalone or tied to a Project module — the flow is slightly different in each case.

I help service businesses automate their operations