How can customer pay online in QB Destktop. I did based on explantion however is not working. It says you have already payment preference. Please help.

Welcome to the Community, @info921. I'm here to walk you through activating your payments so your customers can pay their invoices online. 

First off, you'll need to sign up for QuickBooks Payments by going to the Customers menu, then select Add Credit Card Processing. Follow the on-screen instructions to fill out the form to apply. See article: About QuickBooks Payments.

Once done, turn on the payment options for your customers. Here's how: 

1. Go to the Edit menu, then select Preferences.
2. Select Payments, then choose Company Preferences.
3. In the Online payments section. 
4. Select how you want your customers to pay you online. Then click OK.
5. Select Apply to existing customers.

For future reference, read through this article: Process a credit card payment in QuickBooks Desktop. It helps you learn about processing and recording credit card payment. 

Let me know if you have additional concerns. We're always delighted to help.