Below are the reasons due to which Void Button is disabled on the Payment Workbench form.
1. Payment is Reconciled
When the status of the Payment is 'Reconciled' then the void button will be greyed out. To void a payment, first note the statement number from the show field from the payment then go to cash management and find the statement, and then unreconciled the payment. After this Void Button will be enabled.
2. Prepayment Application / Un-application
If the void button is disabled on a payment and that payment is of Prepayment Invoice then we need to make sure that prepayment is not partially or fully applied. Only after unapplying of prepayment from the invoice, you can void the payment. Until then the void button will be greyed out.