Hello Shivam,
in general altering the table column type from nvarchar to smallint is possible in case the values of the column can be converted to smallint (e.g. no characters, integers in smallint integer range, ...).
But it is not possible if the column is a key column. In that case the type conversion is not possible. As your column is called "ID" i assume that you try to change the type of a primary key column which is not possible.
Regards,
Florian