Hi there @Jo Landers,
I know how important this feature in your business. The reason why you're not able to get a warning because the Routine feature is just replaced by the new feature which is the Auto invoice reminders feature.
You can use the Automate invoice reminders feature since this is just the same set where your QuickBooks Online account will automatically send reminders or invoices to your client. Let me walk you through the steps on how to set up this feature. Here's how:
Set up automatic invoice reminders
Next, craft your message:
For more detailed information, you can read this article to learn more: Automate invoice reminders in QuickBooks Online.
Also, you can pull up a report for your unpaid invoice in your QuickBooks Online account using the open invoice report.
To check the latest product updates in QuickBooks Online, you can always visit this link: The QuickBooks Blog.
If you have follow-up questions, feel free to leave a message in the comment section. I'm always around here in the Community to help. Have a great day!