I've got the details on why you don't have the option to add a custom field on your invoices and what you can do to use this feature, Alex. I'll outline them below.
Please know that the custom field option for QuickBooks Online (QBO) EasyStart (Simple Start) users is primarily dedicated to migrating tags to custom fields only. Although you can add one using this plan, you can't apply it to your transactions. If you want to use this function, you can consider upgrading to the QBO Essentials subscription.
Once you decide to switch your QBO edition to Essentials, this is where you can add/select the custom field from the Invoice page:

For more information about custom fields, check out this link: Add custom fields to sales forms and purchase orders in QuickBooks Online.
You may also personalize your sales forms in QBO. It allows users to modify the available details and design of customer transactions. Access this article to learn more: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Do you have more questions about the custom field feature in QuickBooks? Just click the Reply button and we'll help you out.