A) If you are using the plugin in sandbox mode:
Please make sure you are not using your live account credentials but a separate sandbox account for testing. The testing guide linked below states that you must sign up for it to use the sandbox mode:
https://developer.authorize.net/hello_world/testing_guide/
You can switching the payment mode from the one you have set to the other (sandbox to live or vice versa) in the plugin settings page and see if you see a different error message.
A) If you are using the plugin in production mode:
If you are using live account credentials you will only be able to use the plugin in the production mode and use real credit cards to process an order.
If it does not work in the production mode, it is usually because of an incorrect transaction key. You will need to generate a new one and enter it in the plugin settings and test.