That's an error specific to Authorize.Net checkouts. Everything appears fine with our system and your Authorize.Net integration settings at our end, and other sellers using our system with Authorize.Net are not having problems, so that suggests something to do with your account settings at Authorize.Net's end.
In Authorize.Net's terminology, apparently this "Description" is an extra field for each item (beyond Item Name, Number, Price, etc.) that we do not transmit, so you should make sure that field is not set to "Required" in your Authorize.Net account settings.
I found these steps posted elsewhere that should hopefully get you looking in the right place if they're still accurate:
1. Log into your Merchant Interface at https://account.authorize.net
2. Click Settings link on the left under Accounts.
3. Click Payment Form under the Transaction Format Settings heading.
4. Click Form Fields.
5. Make sure that the Description is not set to required [from what I can tell, setting this to Edit should work okay?]
6. Click Submit to save the settings.