You'll have to connect your Merchant account to your QuickBooks Online so you can start accepting payments through PayPal. I'll be happy to help you accomplish this.
Go to the Gear icon.
Under Your Company, select Accounts and Settings.
Select Billing & Subscription.
Under Payments tab, select Apply now or Connect an existing QuickBooks Payments Account.
Once you've successfully linked your Merchant account, you can set up PayPal as a payment method through your customer's invoice. Just select the Get Set Up link and follow the on-screen instructions to set up your PayPal account.
Additionally, here's a couple of articles you can read to learn more about connecting your Merchant account and accepting customer's payment through PayPal:
The option to connect an existing QuickBooks Payments account is only available if you already had a payment/merchant account. Since you don't have this option, you can click on Apply now to apply for a payment account.
If the problem continues, I recommend getting in touch with our Merchant Support Team. This way, they'll be able to check this further securely. For the support's contact information, you can check it here: Which Payments product do you need help with?.
Stay in touch if you have any other questions. I'm just here to help!