1. Sign in to your Authorize.net account.
  2. Click the Account tab at the top of the screen.
  3. The API Login ID and Transaction Key in this screen, need to precisely match the API Login ID and Transaction Key in Core > Settings > Payment Settings > Select Payment Processing.
  4. Edit the Authorize.net account.
  5. Enter the API Login ID (case-sensitive) in the Account Login field.
  6. Enter the Transaction Key (case-sensitive) in the Password field.
  7. Click Save.