Sorry about that! There’s a bug in the current release that prevents that menu from working correctly. That will be fixed in the next release.
In the mean time, you can create templates by creating and saving any markdown file with the tag template.
Any content of that markdown file (other than the template tag) will be used to populate a new file when you select it.
Until that menu is fixed, you can select existing templates from the file menu > new file from template
Here’s the changelog where that was introduced, with some additional info: https://billiam.itch.io/deepdwn/devlog/309375/v0290-markdown-templates-and-more