Hello, eteklon-may. 

Thanks for reaching out to us here in the Community. I can help you determine why the system keep saying some existing customer is not in the Customer Job list. To verify this, let's check the customer profile. It could be the specific customer mark as Inactive. 

To confirm if this is the case. Let go ahead and mark the customer as Active. Here's how to do it. 

1. Click Customers menu, then select Customer Center. 
2. Under Customers and Jobs tab, tap the drop-down, and choose All Customers.  
3. If the customer name had X, double click the X to unmarked it. 

Should you need some other references in the future, you can check out the following articles below. The re-sort lists can help if you encounter any data list issues in QuickBooks.

• Tracking job costs in QuickBooks Desktop.
• Re-sort lists. 

You can leave a post here in the Community if you have other questions about activating your customer. I'm always here to help. Take care!