I am using the new SubSpace Navigation for Confluence and cannot find the option to create a subspace like in the old version.
How can I proceed?
If you are thinking about ordering spaces hierarchically that is still possible. There are different navigation element types you can use. For your purpose, Confluence space and possibly Folder will work best.
To create a new subspace, edit your menu, hover over your parent space in the edit mode, click "+" and choose "Confluence space".
Choose the Confluence space you want to add as a subspace on the right (field "Confluence space") by starting to type the name and choosing from the dropdown. Even when entering the whole name, you will need to click on the dropdown suggestion.
This step is important or you might get the message "Please fill in all required fields."
Don't forget to save when you're done.
You can add several subspaces on the same or different levels of the hierarchy this way. Use drag-and-drop to move them to different places in your menu or hierarchy.
If you want to group several spaces together but don't want to use a parent space, you can put them in a "Folder". You can then give the folder a descriptive name.