To be able to pay and send your application, every section must be ticked as complete. If you're applying independently, that includes your referee submitting their reference online.
If the payment isn't going through, please double check all details have been entered correctly, you have funds available, and the card has been registered for online payments.
If you're outside of the UK, you may need to let your bank know you're trying to make the payment as well.
Please avoid clicking multiple times as this can also cause problems. If it locks your payment attempts, this will reset automatically after two minutes.