I appreciate you for trying the troubleshooting steps above, eteklon-may.

At this time, I recommend reaching out to our Phone Support team. We have agents there that can do a screen share with you and verify the issue further.

Here's how you can contact them:

1. From the Help menu, select QuickBooks Desktop Help.
2. Scroll down and click Contact Us.
3. Enter The system keep saying some existing customer is not in the Customer Job list as your keyword.
4. Click Continue.
5. You can select how you can contact us.

Once everything's fine, you can check this reference for more information when tracking job costs: Tracking Job Costs in QuickBooks Desktop.

I'll be around if there's anything that I can help. Take care!