Sql server 2008, I want to create non clustered index on 'table variable'

Please see CREATE INDEX (Transact-SQL)[^]
You have to declare the table first, and create your index on it in a second time.

Updated after comment:
I checked, and a temp table is declared with the # sign, not the @ sign (which defines a variable).
Thus:

CREATE TABLE #tmptable (Id int IDENTITY(1,1) PRIMARY KEY, ProductId int)
CREATE NONCLUSTERED INDEX ind1 ON #tmptable (Id, ProductId)

The NONCLUSTERED keyword is optional, as it is the default.

Source:
Create Index on Temp Table[^]

Note: you will have to drop the temp table at the end of your script if you do not want it to persist:

DROP TABLE #tmptable

OR
You can declare an index in a table variable, you just omitted a comma:

DECLARE @tmptable TABLE (
  Id int identity(1,1)
 ,ProductId int
 ,INDEX IX3 NONCLUSTERED(Id,ProductId)
)

Source:
SO: SQL Server : Creating an index on a table variable[^]