The most common reason why you would not be able to void an invoice is that there are existing payments still attached to that invoice. Because voiding an invoice will render the invoice unusable in the future, you will not be able to have any payments attached to the invoice when attempting to void.
Note: Please use caution when voiding an invoice, as this cannot be undone and the invoice would have to be recreated.
Check the Payments portion in the Invoice totals are or the Transaction Details to verify if there are payments attached. You will have to refund the payments by going to Issue Refund under the Invoice Actions menu.