Why I can't manually add a personal trip?

Hello there, lisabrascik-gmai.

When you manually add a trip QuickBooks detects it as a Business mileage since QuickBooks Self-Employed only calculates business trips. You can mark it as Personal if you used the milage auto-tracking feature. 

You can use these resources that will help you handle mileage and taxes:

• Automatically track mileage in QuickBooks Self-Employed
• QuickBooks Self-Employed annual tax guide

Let me know if you have other questions. I'm just around the corner to help you through it. Take care!