Authorize.net returns a merchant login error

Student received error message, more than once, "Merchant login ID or password invalid or account is inactive".  Please advise...

The error means that your login details may not be correct. I would suggest to please generate a new transaction key and use that in the integration to see if that solves your problem.

The following guide should help you: https://support.authorize.net/authkb/index?page=content&id=A682

NOTE: Make sure not to include whitespaces in your username and password.

Please give it a try and let us know if the issue still remains.