Why is the Add field button disabled?

Hello, @akahn.

In QuickBooks Online, the Add field button is only available in QuickBooks Online Essential, Plus and Advanced. If you are using QuickBooks Simple Start, you can consider upgrading your subscription and I am here to show you how.

1. Go to the Gear icon and then select Account and settings.
2. Click the Billing & Subscription tab.
3. In the QuickBooks Online section, click on Upgrade your plan.
4. Click on Choose plan for QuickBooks Online Advanced.
5. Follow the prompts to finish upgrading your plan.

In addition, you can personalize your sales forms that suit your business needs. You can use this article as your guide: Customize invoices, estimates, and sales receipts in QuickBooks Online

If you have further questions about the add field button, you can comment below, and we'll respond to you as soon as possible.