In the Main Content, it is also possible to create new elements graphically, e.g. Main/Sub Processes or Interfaces.
Please make sure to select a view that is intended for modelling beforehand, i.e. Org./Roles Swimlane vertical or horizontal or Vertical / Horizontal.
The other views, such as the Block layout, the Process matrix or the individually configurable Fact sheets, are used for the aggregated presentation of the contents, not for modelling. This is why the New button is greyed out.