This was mentioned here but nothing was done and the thread was closed:
This is still an issue in v11. Inputting either Qty or UOM first does NOT enable the Conversion Factor.
Why is this not set to 1 by default? Under what circumstances would you add a row to a purchase invoice and have the conversion factor set to zero? It just makes no sense. And clearly, a Conversion Factor set to zero is invalid, otherwise it would cough out “Row 1: Conversion Factor is mandatory”.