This article will help you make Authorize.net work in Magento. Magento 2 forces you to use md5 hash, and this hash has to be 10 characters exactly. Also, you need to enable the Transaction Details API in your Authorize.net settings.
To set this up properly:
1. Go here: http://www.miraclesalad.com/webtools/md5.php type in any string of characters such as 'thisisformymagentowebsite' and copy the first 10 characters of the generated hash.
In the example above it would be: 8d603693bc
2. Login to your Authoriz.net account. Go to Account > Settings > Security Settings > MD5-Hash and paste the 10 digits into both boxes.
In Authorize.net you will also need to enable the Transaction Details API. Go to the account >Security Settings > Transaction Details API and enable it, you will need to verify your security question to do so.
3. Go to your Magento 2 admin panel and paste the 10 digit md5 hash in the authorize.net setting.
4. Clear your Magento 2 cache