Vuong Archi:
From the Help files:
Automatic Exterior Dimensioning
Use this function to add automatic associative linear Dimensions to selected Walls or Curtain Walls on the Floor Plan.
So it only works on Walls or Curtain Walls in the floor plan.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David