The Gene Braintree module implements a JavaScript override of the “Place Order now” button on the checkout in order to serialise the card details entered into the Braintree hosted fields - this process returns a unique key (know as a “nonce”) which is then passed to a hidden input field in the checkout form, in turn triggering the checkout form to submit. Upon submission of the form the Gene Braintree payment method passes this nonce to the Braintree API in order to create the transaction.

If this nonce is missing, however, the payment process will fail. This often happens when there is conflicting Javascript or unexpected DOM elements that interrupt the process. Check that your checkout is supported and that there aren't other third-party modules getting in the way of the nonce creation.