Conversion failed when converting the varchar value '5,6' to data type int.

When ever you get data from multivalued checkbox, atleast in SSRS it will come as a String '1,2,3,4,5,6'. You will have to split the string with a function and then you can run the query. Otherwise, the CAST will try to cast 1,2,3,4,5 as INT which is invalid

SELECT *
FROM Table
WHERE Condition IN(SELECT ParsedValues
                    FROM dbo.splitString(inputString))