Are you using an unsupported checkout integration? You can find our list of supported third-party checkouts and their versions here.

If your checkout is a supported one, run a normal checkout test with the browser console open; in this instance, you will often see a Javascript error. Even very small Javascript errors from other modules can affect the payment method - take steps to ensure that your checkout page is error free and then re-test.

If the error is Braintree related, feel free to submit a ticket with the relevant information. If you see no error, submit a ticket with screenshots.