This error message is very common:
It only means that you need to link the Sales Order and the Inventory. For that, go to the "Edit" option in the three-dot menu.
Scroll down until you reach the section "Items".Type the number of the part you want to sell, and a list will unfold. Make sure you select the part from that list.
Click on save at the end of the page, and that's it! Now you'll be able to Enter/Update the Sales Order.