Connect Your Payment Account
Connecting a payment account is what allows customers to pay invoices online. The page for it is the Customer Payments card in Settings, which opens Payment Setup. For a processor you have not connected, choose its Activate Payments button (you pick a business type first) and the processor guides you through creating or signing in to your account. You can instead connect a specific existing account, or, under an advanced option, use a restricted API key.
Once connected you can use Refresh Status, Continue Setup while the processor still needs information from you, and Disconnect. A processor that is connected but not active has a Make Active button (to switch to it without connecting again) and Forget Connection. The connection is to your own account with the processor.
Do this before taking card payments. Payment links and card charges need an active, ready connection, and the Money overview shows the state of the payment account.