Can I enter and opening balance on Customer after the programme is up and running

I have some steps that can help you enter the customer's opening balance, alicelaw.

You can create an invoice by following these steps:

1. On the left panel, click Sales and choose Customers.
2. Find and click the customer's name.
3. Click the New transaction drop-down list and select Invoice.
4. Select the Invoice date and Due date. 
5. Choose Sales from the Product/Service drop-down menu. 
6. Enter the Description as Opening Balance.
7. Enter the Amount, and then click Save and close.

You can also do a journal entry to set up an opening balance. You may wish to ask your accountant for guidance on this.

The following article has more details on what accounts are affected when entering the opening balance: How to set up customer or supplier opening balance.

I'll be around if you need more help with customer transactions in QuickBooks. Thanks.