Thank you for your query,
Yes, this is currently a limitation of the system that costing of the components must be final before assembling. This is due to the fact that the assembled product's average landed cost depends on the component's final cost. As long as the purchase order is still open, the cost is not finalised.
There is currently no ideal way around it but the workaround we recommend is to look at historical purchase order, use the last cost to estimate the cost of the components, assemble the item and once the final invoice comes in, adjust the value of the items.
The limitation of this workaround is that any assembled products that have already been sold would have to be brought back and sold again to correct the margins.
I would like to add a vote on your behalf to support an existing feature request around the recalculation of the average landed cost and the ability to assemble while components are on an open purchase order, please let us know if you'd be happy for me to do so.