I have a customer who wants to pay an invoice with a debit card, how do I activate this tool?

I'll be more than happy to guide you on how you can process customer payments in QuickBooks, thiago-comercial.

With QuickBooks Payments, you can accept credit or debit card, and ACH bank transfer payments from your customers. To sign up, you can follow the steps below.

1. Sign in to QuickBooks Online as an admin.
2. Click the Gear icon and then select Account and Settings.
3. Select Payments. In the QuickBooks Payment section, select Learn more. 

You can also check out this article for more information and detailed steps: Sign Up For QuickBooks Payments. 

Once done, you're customer can now start paying using his debit card.

Use the tools in the Cash Flow menu to track the money coming and out of your business. Here's an article that you can read for your reference: Get Started With The Cash Flow Menu.

Please let me know if you're able to successfully sign up for payments. Don't forget, I'll be right here if you need further help from us. Have a nice day!