Most likely:
- AX does not know where (in the Ledger) to post the product receipt. You'll probably want to fix it by going to the Released product, and looking on the 'Manage costs' fast tab for the item's 'Item group'. Then go to Inventory management -> Setup -> Posting -> Posting. On the 'Purchase order' tab, select the radio button for 'Product receipt', and then provide an account for the Item's Item group.
- If an account is already defined, it may be that the account is not active in the Ledger. Go to G/L -> Setup -> Ledger, and look at the Account structures that have been added. Then goto G/L -> Setup -> Chart of accounts -> Configure account structures, and check that the account that has been defined is in one of the account structures that have been added to the Ledger.