Why can I not assign a unique cutomer ID?

Hi, @user33198. 

While there's no specific field for entering a unique customer ID number to show on printed invoices, I can share a couple of workarounds to achieve this in QuickBooks Online (QBO).

You can consider utilizing the other boxes in the customer details section that you don't use to enter the customer ID number. Alternatively, if you're subscribed to Essentials, Plus, or Advanced, you can choose to create a custom field for the customer ID number. You can follow these steps: 

1. Click on the Cogwheel icon. 
2. Choose Custom fields.
3. Select Add field.
4. Enter the custom field's name. 
5. Put a checkmark for All Sales forms and toggle on Print on form.
6. Click on Save.

However, I understand that the software needs to accommodate various business needs and your experience highlights where it needs to improve. I'd encourage sending your feedback to our product engineers to add a box for entering customer ID numbers in future updates. Here's how:

1. Click on the Cogwheel icon.
2. Select Feedback.
3. Enter your comments and product suggestions about adding a box for a customer ID number. 
4. Select Next to submit feedback.

For future reference, let me add this article as a guide in customising your invoices: Customise invoices, estimates, and sales receipts in QuickBooks Online. 

Keep us posted if you have further concerns about adding a customer ID number to customer info in QBO. We'll do our best to assist. Take care.