Ok, here's how you can get this pretty easily. You are going to have to be careful which groups you are choosing filter parameters from. This has burned me recently because I apparently wasn't paying attention to which group the filter was being pulled from. Here's how to create the report: 

• Add a new report and select Custom Report. 
• In Category, select Workspace ONE UEM then Devices
• Give your report a name so you can save it
• In the rules, type in "platform" in the Select Attribute area
• MAKE SURE you choose "Platform (Normalized)" from the Device group. If you select it from the default Apps group, you will not get the results you want. 
• Add "Windows Desktop" as the platform after the "includes" parameter.
• Refresh the Report Preview to see how many Windows Desktop records you have.
• Next, add a new rule and choose Devices > OS Version (Normalized) and make the rule reads like this:
  • OS Version does not include (10.0.19044)
• Refresh the Report Preview and that will be the number of Windows Desktop devices that do not have the most current GA release of Win10. 

Hope that helps, 
Joel