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. Here is a help document for your reference:
https://experienceleague.adobe.com/docs/experience-manager-64/assets/dynamic/managin...