Why can't I edit my material item's unit cost?
If it's a managed item, you cannot directly edit its unit cost because the item value is tracked in Xero. If you need to update the unit cost because your supplier has updated their pricing, you can add the new unit cost in the catalogue price field. When you make a purchase at the catalogue price value, the item's overall unit cost will update as a weighted average.
See How to use catalogue pricing for more information.