The reason for calculation failure. If the calculation fails, this message will be shown in an error, or warning if we are editing the constraints. 
Namespace: Autodesk.Revit.DB.Structure
Assembly: RevitAPI (in RevitAPI.dll) Version: 24.0.0.0 (24.0.0.0)
Since:  2018.1 
Syntax
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Exceptions
| Exception | Condition | 
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| Autodesk.Revit.ExceptionsArgumentException | When setting this property: errorMessage is an empty string. | 
| Autodesk.Revit.ExceptionsArgumentNullException | When setting this property: A non-optional argument was null |