Magento 2 comes with Authorize.net direct post as a payment method. Mostly due to the response url that Magento posts to transact.dll causing issues. You can debug those issues for a long time and eventually fix them, but it hardly seems worth it.
For the longest time, Authorize.net and Magento were a safe combination, but their push for Braintree seems to be changing that.
The last two sites I’ve worked on that ran Magento 2 for the platform and Authorize.net as the payment gateway had issues with checkout.
At this point, I would highly recommend using this extension to handle authorize.net credit card transactions.
It will work even if you don’t want to offer customers the ability to save credit cards and all of the issues with direct post and the response URL bugs. It has saved me countless hours of debugging.