Welcome and thanks for visiting the Community. Currently, the Pay now option is only available when you create an invoice and send it to your customers so they can pay you online.
To do this, you'll need to sign-up for Online Payment first. The decision will take up to three (3) business days. Once approved, you'll receive an email with instructions on how to set it up with your QuickBooks Self-employed (QBSE) account.
See this article: Let your clients pay you online for more detailed steps about applying for QuickBooks Online Payments. It also includes contact information if you have more questions about your payments application.