1. Warning message displays if the current selection contains elements that cannot be included in an assembly.
The following elements cannot be included in an assembly:
  • Annotations and detail items
  • Assemblies and elements that are already part of another assembly
  • Complex structures (trusses, beam systems, curtain systems, curtain walls, stacked walls)
  • Elements in different design options
  • Groups
  • Imports
  • Images
  • Links or elements in a link
  • Masses
  • MEP-specific elements (ducts, pipes, conduits, cable trays and fittings, HVAC zones)
  • Model lines
  • Rooms
  • Structural loads, load cases, and internal loads

See the rest at:
Assembly Usage Tips – WikiHelp