Why Are Some Sheets in Revit Not Loadable?

Sheet or Drafting View is not loadable due to non-loadable elements

Sheets in Revit may not be loadable because they contain model-specific elements on the sheet (plans, sections, elevations, 3d views, legends).

At this time, Revit is not able to load sheets with those types of elements on them. Removing those elements from the sheet should allow them to be loadable.