Thanks for posting in the Community, @Sconner1. Let's perform some troubleshooting steps so you can create estimates for existing customers using the QuickBooks Online (QBO) mobile app.

In the QBO mobile app, you can enter estimates for already-added customers. Since you're unable to do so, let's troubleshoot your mobile app to resolve this issue.

Here are some quick steps:

1. Change the Wifi data connection on your device to 4G (varies by device).
2. Restart the app.
3. Clear app data. To do this:
   1. Open your device Settings.
   2. Select Apps (varies by device).
   3. Select the QuickBooks app from the list, then Storage.
   4. Tap clear data.
4. Restart your device.
5. Uninstall and reinstall the app.

Moreover, I'll share this article to help you make quick updates to your invoice template: Customize invoices on the QuickBooks Online mobile app.

Keep in touch if you have other questions about recording estimates in QBO. We're always here to help. Take care, and have a good one.