You're right in applying a journal entry to an invoice, @growingforgood.

Once you've entered a journal entry crediting to the Accounts Receivable account, this automatically applies to the open invoice when recording a payment. For now, let's review why you're getting this error message. Make sure that the transactions have the amount for both the invoice and the journal entry.

I've added these screenshots for your visual reference.

This article also details the steps about applying a journal entry credit to an invoice in QuickBooks Online.

I'm sure you'll now be able to record the transactions accordingly. If you have any other questions, mention me in the comment section below. I'll get back to you as quickly as possible!