Thank you for your prompt reply, Tommy. I appreciate the screenshot you've added, and I want to make sure you'll be able to add a custom field to your forms in QuickBooks Online (QBO).

The Custom fields or Add field tab isn't available once you don't have permission on the said feature. To isolate the issue, please sign out to QuickBooks, then reopen it as a master admin or user access with rights to adding and customizing fields.

Once done, you can check out this link to learn more about creating your own fields for your invoices, and other transactions: How to add custom fields to sales forms and purchase orders.

In case you're getting the same result, I recommend replicating the steps using a private or another supported browser to narrow down the result. It's the best place to check if the issue is within the browser you're using.

If you have any other questions about custom fields, let me know by adding a comment below. I'm always here to help. Keep safe!