Hi Anne Anita,
You need to check that Default Warehouse is Selected in Item Master Data or Not.
You can check Warehouse in Item Master Data >>> Inventory Tab
If Default Warehouse is not Selected in Item Master Data then you usually get this error.
But you can continue your work by Selecting Correct Warehouse which Warehouse is Selected in Item Master Data >>> Inventory Tab
Hope this helps 
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Regards::::
Atul Chakraborty