Voiding an invoice is disabled if the are payments (including credit notes) attached to the invoice, i.e. the outstanding amount on the invoice is not equal to the invoice total.
A credit note on an invoice is treated as payment and the outstanding amount of the invoice is reduced by the amount of the credit note,
An invoice with an attached credit note can be voided by firstly voiding the attached credit note and/or all payments of the invoice to be voided.