Yes, you must show the 20% CIS deduction on your invoice. As a subcontractor, you should invoice the contractor for the full amount (gross) for labor and materials, and then clearly show the 20% CIS deduction applied to the labor portion, resulting in a lower net amount due. The error message you're encountering means the builder is not set up correctly as a CIS contractor in your customer settings.
To enable CIS functionality, edit your customer profile by heading to All apps on the left navigation, choosing Customer & leads under Customer Hub, clicking Edit, and in the Additional info section, check the Is CIS contractor box.
Once these settings are updated, you can go back to your invoice. You will now be able to select CIS Labor as an item. The software will automatically calculate the 20% deduction for you, showing the gross amount, the deduction, and the final net amount you expect to receive in your bank account.
I hope this helps you get that first invoice sent off without any further issues! If you run into any more trouble with your CIS settings or need a hand balancing your labor and materials, just let me know. I look forward to assisting you further.