Good Afternoon, @ytguo.
I hope you're having a great weekend so far. The good news is, you've reached the right place and I have the tools to help. Let me show you how to turn on the reminder email setting within your QuickBooks account.
The answer to your second question, if you wish to send the reminder, you'll need to do so by going to the invoice. Here's how:
It's a piece of cake! To learn more about invoice reminders, look into this guide for details.
Should you need any further assistance, don't hesitate to ask. I'm always here to lend a helping hand. Enjoy the rest of your weekend!