InDesign files get the preview from a plugin that the user who creates the file needs to have installed on their machine at the moment of saving the file.
The TeleScope plugin for Indesign 2020 is available on the Adobe Exchange website. Make sure this is installed on your local machine for TeleScope to be able to generate previews of your INDD files. If you're using the latest version of an Adobe product and there is not yet a supported version of a plugin, you can use the last released version of the plugin.
You can search for TeleScope plugins for other Adobe products here.
Once a suitable INDD plugin is installed, the generated files when uploaded to TeleScope will have previews generated.