As pointed out already in the comments by marc_s, IN
requires a list of values not simply one variable.
But you could provide those values in a SELECT
from a simple table variable
like this:
DECLARE @Param1 TABLE (Value NVARCHAR(255));
INSERT INTO @Param1 (Value)
SELECT 'Test1'
UNION
SELECT 'Test2'
UNION
SELECT 'Test3'
SELECT
*
FROM TABLE1
WHERE
COLUMN1 IN
(SELECT Value FROM @Param1)