Inventory Valuation Report shows zero Average Cost and Last Purchase Price for all locations, Item record displays values.
The ability to show the average cost and last purchase price for all locations even without setting preferred location on items is currently covered by Enhancement 156866.
When Multi- Location Inventory (MLI) feature is enabled, Average Cost is displayed only for the item's preferred location. If there is no preferred location defined for an item, the average cost and last purchase price displayed on the report is blank or zero.
As an alternate solution, you may set a Preferred Location on the item record.