RE: Inventory dimension Site is mandatory and must consequently be specified.
While this is an old thread - I was looking for an answer and didn't find it here but managed on my own.
First off of course this comes if you haven't specified site on sales order lines tab: product field Site (values defaulted from header where it is defaulted from the customer)
My issue was because I was setting up inter company and you need to ensure that you have all the default values on both the vendor and the customer. including other fields such as language. In order to ensure that IC is setup is setup correctly try and create both a purchase order and a sales order to see if they each on their own comes out with all the necessary values