applying credits to invoices, why doesn't message appear when creating invoice that the customer has a credit

Hello there, @ymarsden.You can turn on one-time messages in QuickBooks Desktop to get a reminder for applying credits whenever you're making an invoice. Just tweak the Preferences a bit.

To set the Preferences, click to the Edit tab > Preferences > General. Under My Preferences, tick the checkbox for Bring back all one-time messages. See the sample screenshots below.

Don't hesitate to comment below if you have additional questions about enabling one-time messages in QuickBooks Desktop.