I set up a sample site, using the following values and it worked correctly:
Payment gateway url: https://test.authorize.net/gateway/transact.dll
API Login: 92KgL2uD
API Key: 4nT73z4473vLJfXF
Test Mode: TRUE
You can try using that login / key (they are test ones anyway) and see if it still has a problem. If it does get an error, you must have an incorrect configuration somewhere. If it doesn't get the error, your Authorize.NET account must not be configured correctly and you will need to reach out to Authroize.NET support.
You probably already know this, but just in case...
The Kentico documentation provides steps to configure for Authorize.NET here.
Hope this helps!