RE: Inventory dimension Site is mandatory, Intercompany purchase/sales order
This is probably quite late to reply to this question.
However, I am putting my response here, just incase someone faces it in future and comes looking for the answer here. The error message is not very helpful, and probably AX should try to make them clearer.
The error you are getting is probably due to the storage dimensions setup on the customer/ vendor. By default, this is blank, but then AX has made site mandatory. Inorder to resolve this, go to the customer/ vendor card of the respective companies, go to the Sales Order defaults (or Purchase Order defaults) and choose a Site as default from the list.
Next time you create a Purchase order, the corresponding sales order in the sister company will use this default Site value, and hopefully will not throw this error message.
Hope this helps.