I have been telling people for years that Revit needs at least one level to function. As proof, if you only have one level you'll find that Revit won't let you delete it, the last one. The Delete button is disabled and it ignores you if you tap the Delete key.
I find it disturbing and interesting that it IS possible to delete all the levels as long as you select all of them and delete them together, at the same time. No complaint from Revit at all.
That seems like more like a bug than a feature to me. It works this way as far back as v2013, as far back as I've tested it.
What if you've done that, deleted all of them? You'll probably be a bit disappointed at the condition of what was your model...but just open an elevation view and create a new level(s).