To make the prepaid invoice services of membership assigned, you need to clear the GST or need to adjust the prepaid amount which is less than or equal to taxable amount:
To adjust the prepaid amount:
Go to the "Quick sale".
Select the Client for whom you want to create the invoice.
Select the services, staff and time.
Adjust the amount that is equal to the sub total and less than taxable amount.
And Select the "Prepaid" as a payment mode.
Click on the "Generate Bill".
Or you can also simply remove the GST amount, Select the "Prepaid" as a payment mode and generate the bill.
