Why am I unable to create Interest-only Invoices?

Interest-only Invoices get generated if the Invoice fits the Client-level, Matter-level, and Invoice defined interest rules.

You also need to ensure that if the Billing Category is specified under the Clients and Matters>Client Details>Invoice Settings, it is selected when generating the Draft.

Also, if the Interest Start Date is specified under the Clients and Matters>Matter Details>Invoice Settings, then the Interest-only invoices will be generated after the specified date.