Thanks for reaching out. Have you applied for a merchant services account? In order to be able to take payments through QuickBooks Online, you would have to apply for a merchant services account and receive confirmation that the account has been approved.
Here is some information on the merchant services we offer for QuickBooks Online:
You can either apply online through that link, reach out to one of our sales' agents through the phone number at the top of that page, or apply in-product by going to the gear icon, account and settings, click on the payments tab, and then select learn more.