You need to first create a new Stripe account or use an existing account and connect to it from Global Settings > ePayments. You also need to create or have a Plaid.com account for bank verification. Then you need to define the online payment settings there. Check out Core Help Center > Global Settings for details. After the setup, the ePayments option is enabled for all new invoices. You can enable this option for existing invoices from the Invoices screen. Check Core Help Center > Invoices for details.