If you are marking an invoice as paid for the first time, this option may show as unavailable with the message "Payment Account defaults must be set in Integration Settings page."
If you see this, it means that no payment accounts have been made available to assign the payment to.
To make a payment account available, go to the Integrate screen. Then click on the Add Payment Account button on the right of the screen and choose a payment account to make available.
If when you click the Add Payment Account button you see a message saying "All payment accounts are added", please check that payment accounts have been set up in your accounting package. If these have been set up successfully, please click the Resync Data button, allow 5-10 minutes for this resync to complete, and then try again.