User is trying to pay the outstanding invoices on a project from the Payments screen, but cannot find the project in the drop-downs there.
This can happen if the project has a status other than Active. By default, Core does not show projects that have a status other than Active in the drop-downs. As a workaround, you can see the outstanding invoices on such projects by selecting the Client in the Add Payment dialog and leaving the Project field blank. Core displays all outstanding invoices of the client irrespective of the project's status. You can then pay off those invoices as required.
Note: You can also pay off the invoices of such projects from the Invoices screen itself, as it shows all invoices irrespective of project status. Check the Core Help for more on that.