There are a couple of "issues".

• there is a parenthesis missing at the start of the Expression code block
• The DatastoreUrl property points to a VirtualMachineConfigInfoDatastoreUrlPair object, which has multiple properties. If you don't select the property, the Select-Object don't know how to display the values
• A VM can have multiple datastores. To cope with that (especially when you to do an Export-Csv or similar), join the values together into one string

Try like this