The first symptom that your STL has geometry errors comes from the mesh to solid conversion process in DSM. If the object remains a surface after a conversion attempt, it means that there are problems with how certain faces or edges connect.
Although DSM does not have a repair option built-in, you can do the following:
1. In DSM 4.0: Click on the 'export to 3D hubs service' in the design tab and upload your STL on the webpage. Look at the right hand side of screen for Mesh Integrity and Printability process checks.
If a red caution sign is flagged, there are a number of faults in the mesh which break the slicing process and therefore cannot be printed.
2. On Windows 8.1 and 10, you have access to a free app from the Windows store called '3D Builder'. Download this if you don't have it pre-installed: https://www.microsoft.com/en-us/store/p/3d-builder/9wzdncrfj3t6
3. In MS 3D Builder, load the faulty STL. It will automatically detect the faults and prompt for the fixes to run. Click on the notification prompt to complete repair.
Remember to save the changes to the STL after repair.
The developer of DSM (ANSYS SpaceClaim) also offers seamless STL repair options and a full featured 3D modelling suite in the paid upgrade option. Check here: http://www.spaceclaim.com/en/ANSYS-SpaceClaim-trial-DSM-UI-campaign.aspx
or, click on the Help/Resources-> Upgrade CAD option inside of DSM 4.0