Getting the following error when checking a challenge. I have tried with 2 dev orgs. Anyone know how to fix this?

There was an unhandled exception. Please reference ID: RNFMDUAP. Error: Faraday::ClientError. Message: INVALID_FIELD: SELECT Title, ArticleBody__c FROM Knowledge__kav ^ ERROR at Row:1:Column:15 No such column 'ArticleBody__c' on entity 'Knowledge__kav'. If you are attempting to use a custom field, be sure to append the '__c' after the custom field name. Please reference your WSDL or the describe call for the appropriate names.

i have one picklist of products and a price field to give price when a product is selected,,
i want to write a validation rule so that when product is selected price have to be given and vice versa ,if product is not selected ,we should not enter price)

can anyone provide me a validation rule for this without using is related functions
Thanks in advance
Prabhat Jha