ALTER TABLE dbo.Whatever ADD ComputedColumnName AS CONVERT(VARCHAR(500), COALESCE([fieldValue],'')) PERSISTED; 

You won't be able to index any more than 900 bytes, so with NVARCHAR, that means the first 450 characters. The important point is that using LEFT on its own does not set the destination data type, you need to do that explicitly using CAST or CONVERT, e.g.

ALTER TABLE dbo.Whatever ADD ComputedColumnName AS CONVERT(NVARCHAR(450), COALESCE([fieldValue],'')) PERSISTED;