You are removing an element from the group which is involved in a sketch or family constraint with another element of the same group. Either remove the constraint or cancel the removal of this element from the group. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
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