I had previously been able to create subforums, but not sure of software config when I did so.
It happened with all versions of elementor available to , up thru 2.0.7 (this morning)
But you can close this ticket.
Afte extensive testing, I was able to trace the problem as follows:
The parent-child feature breaks when activating elementor to allow editing of forum post types. When I disabled this capability, it started working correctly again.
I don’t know if this affects other post types, or if allowing other page builders to edit forum post types would cause the same problem. I leave the second issue in your hands, in case you find additional form builders with the same problem. I suppose it is easy enough to test if you have other builders available such as Beaver that allow various post types.