There are two scenarios you can consider why you're not seeing a payment option in your invoices. I'm here to share them and help you from there.
First off, if you're the owner of the account and you're sending an invoice to your client, you'll to enroll first to the QuickBooks Merchant service. From there, you can start sending an invoice and let your clients pay invoices online.
However, if you're a client who's about to pay our invoice online, the owner of the account who sent you the invoice must be enrolled to the QuickBooks Merchant Services.
Thus said, you'll have to contact the one who sent you the invoice to confirm why you're unable to pay your invoice online.