There are a few things that may cause an error.
1. The authorize.net login information entered into your Site Settings page is incorrect.
2. The liveSite Payment Gateway mode is set to 'Test' and the authorize.net account is set to 'live' (or vice-versa).
3. The authorize.net transaction key has expired. In this case you will need to request a new transaction key from your authorize.net control panel.
4. You are using an Authorize.Net test/developer account, which liveSite does not support.