Please try re-configuring the AEM DM instance and ensure that you have the publish set to immediately for the first time, once setup is complete, you can make changes as per your liking. After completion you should see the "/conf/global/settings/dam/dm/presets/viewer" in crx/de.
After which you can go to tools > Assets > viewers card and create a viewer of your choice.
When you configure dynamic media cloud service, OOTB viewer presets present under /libs/settings/dam/dm/presets/viewer get copied to /conf/global/settings/dam/dm/presets/viewer after which they are seen on the viewer presets page. So, please check that you have created DM cloud configuration.