Void option on Payment actions screen is disabled while trying to void a payment.
Oracle application R12(12.1.3)
The payment is done for a prepayment and the prepayment is already applied to a standard invoice.
Oracle actually restrict voiding a payment for a prepayment already applied to an invoice
To void the payment
Go to prepayment applications and unapply any invoices applied
Query for the invoice to which you have applied the prepayment (now the invoice should have new distribution line and status must have changed to ‘Needs revalidation’)
Validate the invoice and create accounting
Go back to payment screen and query the payment you want to void
Click the Actions Button
Void check box should be enabled now