Yes, you're allowed to set up 3 custom fields in your sales forms, domgee.
Once you change your custom field to include the SKU, you still need to manually enter them in your invoices.
I know how important it is to automatically include the sku number in the SKU field to keep track of your transactions. Since you're unable to auto-populate SKU after performing the steps provided by my colleagues above, I'd recommend contacting our QuickBooks Online Support team. They can identify the cause why your SKU codes will not automatically appear in your invoices and help you fix it.
You can check out this article to learn more about the different types of support we offer at Intuit and its availability: Support hours and types. It also contains important reminders about occasional downtime in the Footnotes section.
Additionally, you can pull up the Transaction Detail by Account report to view the list of all your invoices that are due on a particular date range. For the detailed steps, you can refer to this article: Invoice by due date report.
Please let me know if you have other concerns. I'm just around to help.
