It is possible to end up in a state (possibly due to bounced email addresses) where you have no Active contacts on a payment or invoice.
In this state, replies are disabled on the payment page in Routable, because there are no valid contacts available to receive the message.
To resolve this issue and enable replies on your payment or invoice, click on the Vendor/Customer tab in the right side panel on the payment page. In this tab there is a section called Contacts on this payment with a button to add a new contact to the payment or invoice.
Clicking this button gives you the option to search for and add an existing contact, or you can create a new contact.
Once a new contact has been added to the payment or invoice, the reply editor will be enabled and you will be able to send a message to the new contact.