Microsoft sql server compute column

I have a microsoft sql server table with a computed column named “RowOrder”:

CREATE TABLE [dbo].[Miss](
	[ID] [int] IDENTITY(1,1) NOT NULL,
	[RifID_Soc] [int] NOT NULL,
	[Data] [date] NOT NULL,
	[Note] [nvarchar](max) NULL,
	[Stato] [char](1) NOT NULL,
	[Annullata] [bit] NOT NULL,
	[RowOrder]  AS (case [Stato] when 'I' then (1) when 'P' then (2) else (9) end) PERSISTED NOT NULL,

In Cuba Data Model this column is defined as Read Only.
In the Edit Screen, when I create a new record I receive an error:

SQLServerException: The column "RowOrder" cannot be modified because it is either a computed column or is the result of a UNION operator

Because Cuba want to insert a value for this column even if it is readonly.

Thanks for your help