Update your software. You have a very old version.
Also, what you want can be obtained from the Inventory Quantity Summary under Reports tab. You don’t need a custom report. If you have at least one inventory location report defined, it can also be obtained in the Inventory Quantity by Location report.
You can also export the Inventory Items tab listing to a spreadsheet to obtain this and other information.
In fact, the one thing you cannot do to obtain this information is generate a custom report. Inventory quantities are the result of processing prior transactions involving inventory items. Custom reports only return information about transactions themselves, not balances.