Below script will work in this case or not

CREATE TABLE TEST_UQ (COL1 INT IDENTITY(1,1) PRIMARY KEY, COL2 VARCHAR(10) NULL,COL3 AS (CASE WHEN COL2 IS NULL THEN CAST(COL1 AS VARCHAR(10)) ELSE COL2 END))

GO

CREATE UNIQUE INDEX TEST_UQ_IND_1 ON TEST_UQ (COL3)

GO

This also works for in my case