Answer: 

A receipted status for a Purchase Order allows final costing to still be added, which means Average Landed Costs of products on that Purchase Order may need re-calculation.

Since the cost of an assembly is auto-calculated from the Average Landed Cost of the component products, you cannot complete an Assembly until the final Landed Cost is computed i.e when the receipted Purchase Order is completed.  

Additional Information:

• To sell the receipted products without creating any assemblies for it, use the Purchase Order re-costing functionality. Discard the assembly and add the final product via a Stock Adjustment.
• If the assembly needs to be completed, the Purchase Order must be completed.
  • When the Purchase Order invoice arrives at a later date and there is remaining stock after the Assembly has been completed, perform a value-only stock adjustment to accurately reflect the landed cost of the received products. 
    • Value changes are applied moving forward and will not affect any historic transactions or data for those products.  
    • Will depend on whether or not you have any stock on hand left for those products when their final purchase invoice arrives.
    • Will not export any purchase cost bills to your accounting system for you to pay off. We will only export a Stock Adjustment journal to update the stock on hand value with the amount you manually adjusted.
    • See What is a Value-Only Stock Adjustment? for more information.