This is because the prepaid item in this ticket already used or the outstanding balance for this ticket already paid. So if you want to void this ticket, you may check which case can apply to this ticket.

CASE 1:
“If the prepaid item already used in other tickets.”

Step 1: So you may need to find out which ticket the customer consumes the prepaid item
Step 2: Edit the ticket and delete the item / void the ticket
Step 3: After that go back to the ticket and void it

CASE 2:
“If the outstanding balance already paid.”

Step 1: You may go to Payment and check which day and which record the customer paid the balance
Step 2: Click Edit and void the record
Step 3: After that, you may go back to the ticket and void it