Once you have enabled Online Payments to your website, you are then able to decline registrations if a payment is unsuccessful.
This is useful if you only want registrants to appear on the database who have paid there fee's.
To set a mandatory payment, simply edit the form you wish to set the mandatory payment to, then scroll to the bottom of the form to the heading Products.
Tick the box "Decline Registration if Payment Unsuccessful".
If someone elects to use Sport$pay to pay you online by direct debit but they have insufficient funds in their account, it is treated in a similar way to someone writing you a cheque with insufficient funds (ie their cheque would bounce). Their own bank may decide to charge them a dishonour fee. If their once-off direct debit payment fails due to insufficient funds, you will need to contact the person directly to organise payment in another way or ask them to follow the payment link in the automated confirmation email they received when they registered, to make payment afresh once they have sufficient funds available.
If you would like to know how to set mandatory payments on an individual product, please see https://support.sportsground.com/hc/en-us/articles/360015740951-Set-mandatory-payment-on-individual-products-on-a-form.