We can add items to an invoice transaction when entering SKU numbers, 12345CP.

In QuickBooks Online, the SKU option is available in all online versions. You should be able to add your product or service items via SKU numbers to your sales forms. I've added a screenshot for visual reference.

Adding items to an invoice via SKU number in an old form layout:

Adding items to an invoice via SKU number in a new form layout:

However, if some users are unable to add items to the customer sales transaction, troubleshooting and refreshing your company data may help rule out the possibility of a browser issue. Refer to these keyboard shortcuts to open one:

• Google Chrome: Ctrl + Shift + N.
• Mozilla Firefox and Microsoft Edge: Ctrl + Shift + P.
• Safari: Command + Shift + N.

You can also try opening your account in a different browser. This could be an issue with the browser that you're using. Once it works from there, go back to your regular browser and clear its cache.

Moreover, you may utilize this reference to help you track and enter customer-paid invoices: Record invoice payments in QuickBooks Online.

You're always welcome to reply to this post if there's anything else you need further assistance managing customer transactions in your file. I'll be around whenever you require additional help. Take care and have a good one!